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]

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 ⚠️

Additional details and impacted files

@@            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	Coverage Δ
unittests-3.10	96.74% <62.50%> (-0.50%)	⬇️
unittests-3.11	96.74% <62.50%> (-0.50%)	⬇️
unittests-3.12	96.74% <62.50%> (-0.50%)	⬇️
unittests-3.8	96.74% <62.50%> (-0.50%)	⬇️
unittests-3.9	96.74% <62.50%> (-0.50%)	⬇️

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