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',

yadm

@@ -175,6 +175,10 @@ function score_file() {
     label=${field%%.*}
     value=${field#*.}
     [ "$field" = "$label" ] && value="" # when .value is omitted
+    # extension isn't a condition and doesn't affect the score
+    if [[ "$label" =~ ^(e|extension)$ ]]; then
+      continue
+    fi
     score=$((score + 1000))
     # default condition
     if [[ "$label" =~ ^(default)$ ]]; then

yadm.1

@@ -526,6 +526,11 @@ and trimming off any domain.
 .TP
 .B default
 Valid when no other alternate is valid.
+.TP
+.BR extension , " e
+A special "condition" that doesn't affect the selection process. Its purpose is
+instead to allow the alternate file to end with a certain extension to
+e.g. make editors highlight the content properly.
 .LP
 
 .BR NOTE :