Add support for edited statuses

[bekriebel]

ActivityPub supports "status.update" for editing statuses. These should be made available for streams.

[codecov-commenter]

Codecov Report

Base: 87.63% // Head: 87.77% // Increases project coverage by +0.14% :tada:

Coverage data is based on head (620be36) compared to base (29bb160). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #170 +/- ## ========================================== + Coverage 87.63% 87.77% +0.14% ========================================== Files 15 15 Lines 954 965 +11 ========================================== + Hits 836 847 +11 Misses 74 74 Partials 44 44 ``` | [Impacted Files](https://codecov.io/gh/mattn/go-mastodon/pull/170?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn) | Coverage Δ | | |---|---|---| | [streaming.go](https://codecov.io/gh/mattn/go-mastodon/pull/170/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn#diff-c3RyZWFtaW5nLmdv) | `97.89% <100.00%> (+0.14%)` | :arrow_up: | | [streaming\_ws.go](https://codecov.io/gh/mattn/go-mastodon/pull/170/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn#diff-c3RyZWFtaW5nX3dzLmdv) | `96.84% <100.00%> (+0.17%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattn)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[mattn]

Thank you