Initial version of sending from a file

[mickel8]

Things that are still missing:

• SSRC is hardcoded
• we don't check whether the other side didn't reject our mline
• we need to update extension ids if the other side sends its offer first
• unit tests

[codecov[bot]]

Codecov Report

Merging #30 (a0379ba) into master (8693629) will increase coverage by 0.69%. The diff coverage is 95.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #30 +/- ## ========================================== + Coverage 84.43% 85.13% +0.69% ========================================== Files 12 14 +2 Lines 559 592 +33 ========================================== + Hits 472 504 +32 - Misses 87 88 +1 ``` | [Files](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc) | Coverage Δ | | |---|---|---| | [lib/ex\_webrtc/peer\_connection/configuration.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9wZWVyX2Nvbm5lY3Rpb24vY29uZmlndXJhdGlvbi5leA==) | `82.35% <ø> (ø)` | | | [lib/ex\_webrtc/rtp/vp8\_payloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvdnA4X3BheWxvYWRlci5leA==) | `100.00% <100.00%> (ø)` | | | [lib/ex\_webrtc/rtp\_sender.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfc2VuZGVyLmV4) | `100.00% <100.00%> (ø)` | | | [lib/ex\_webrtc/rtp\_transceiver.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfdHJhbnNjZWl2ZXIuZXg=) | `92.59% <100.00%> (+0.28%)` | :arrow_up: | | [lib/ex\_webrtc/peer\_connection.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9wZWVyX2Nvbm5lY3Rpb24uZXg=) | `80.93% <86.66%> (+0.06%)` | :arrow_up: | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). Last update [8693629...a0379ba](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/30?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc).