Support non-http schemas in url refs

[alts]

These url schemes were already valid for inline linking. This adds
the ability to use them as url references as well.

[codecov]

Codecov Report

Merging #75 (6b787c9) into master (23e7746) will increase coverage by 1.16%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #75      +/-   ##
===========================================
+ Coverage   98.83%   100.00%   +1.16%     
===========================================
  Files          11        12       +1     
  Lines        1206      1199       -7     
  Branches      221       222       +1     
===========================================
+ Hits         1192      1199       +7     
+ Misses         14         0      -14     

Impacted Files	Coverage Δ
textile/core.py	100.00% <100.00%> (+1.88%)	⬆️
textile/__init__.py	100.00% <0.00%> (ø)

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[coveralls]

Coverage remained the same at 100.0% when pulling 6b787c9 on alts:url-ref-schemas into 23e7746 on textile:master.

[sebix]

The change itself is fine for me, thanks for the contribution.

But I see, that the regular expression pattern is compiled on every function call, we can move this to the constructor, where all the other patterns are compiled as well, to gain some performance.

[alts]

But I see, that the regular expression pattern is compiled on every function call, we can move this to the constructor, where all the other patterns are compiled as well, to gain some performance.

I'll add that change as well. Thank you.