internal_links check for xhtml files

[lo48576]

internal_links checker checks *.html files, but not *.xhtml files.

Steps to reproduce

1. Prepare (generate) *.xhtml files with wrong site-local links.
2. Run nanoc check ilinks

Expected behavior

nanoc check ilinks detects broken links in *.xhtml files.

Actual behavior

nanoc check ilinks doesn't detect broken links in *.xhtml files.

Details

Maybe this line should be changed:

nanoc/nanoc/lib/nanoc/checking/checks/internal_links.rb

Line 18 in 9050195

filenames = output_filenames.select { |f| File.extname(f) == '.html' }

I'm not sure this should be configurable or can be hard-coded, but I think not only *.html but also *.xhtml and *.htm should be (can be) checked as HTML file.

[denisdefreyne]

Good catch! Here’s a fix: #1329