Add support for the extension "condition"

This special condition doesn't affect the selection process and is only
supported to support syntax highlighting and language detection for alt files
as reported in #239.

test/test_alt.py

@@ -81,6 +81,7 @@ def test_relative_link(runner, paths, yadm_alt):
 @pytest.mark.usefixtures('ds1_copy')
 @pytest.mark.parametrize('suffix', [
     '##default',
+    '##default,e.txt', '##default,extension.txt',
     '##o.$tst_sys', '##os.$tst_sys',
     '##d.$tst_distro', '##distro.$tst_distro',
     '##c.$tst_class', '##class.$tst_class',