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.

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