Ignore invalid URLs that cause urljoin to raise.

[jyasskin]

This just copies the invalid URL into the markdown link verbatim instead of trying to join it to the base URL.

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (099c4b8) 97.23% compared to head (4157f6f) 96.74%.

Files	Patch %	Lines
html2text/__init__.py	62.50%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #393 +/- ## ========================================== - Coverage 97.23% 96.74% -0.50% ========================================== Files 11 11 Lines 1122 1135 +13 ========================================== + Hits 1091 1098 +7 - Misses 31 37 +6 ``` | [Flag](https://app.codecov.io/gh/Alir3z4/html2text/pull/393/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand) | Coverage Δ | | |---|---|---| | [unittests-3.10](https://app.codecov.io/gh/Alir3z4/html2text/pull/393/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand) | `96.74% <62.50%> (-0.50%)` | :arrow_down: | | [unittests-3.11](https://app.codecov.io/gh/Alir3z4/html2text/pull/393/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand) | `96.74% <62.50%> (-0.50%)` | :arrow_down: | | [unittests-3.12](https://app.codecov.io/gh/Alir3z4/html2text/pull/393/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand) | `96.74% <62.50%> (-0.50%)` | :arrow_down: | | [unittests-3.8](https://app.codecov.io/gh/Alir3z4/html2text/pull/393/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand) | `96.74% <62.50%> (-0.50%)` | :arrow_down: | | [unittests-3.9](https://app.codecov.io/gh/Alir3z4/html2text/pull/393/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand) | `96.74% <62.50%> (-0.50%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alireza+Savand#carryforward-flags-in-the-pull-request-comment) to find out more.

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