fix wmltools' parse_attribute to not consider pango color hashes as a comment

While there are many places that wmllint assumes that "#" begins a comment, from Vultraz's description it sounded like parse_attribute was being used to find the attribute's value, and then a string_strip was done on the value. So I looked for the direct cause in wmltools.

Again, the solution is to look for whitespace to precede the hashsign.

Incidentally, I don't know why the original code had the first "where -= 1", then had value and comment go from [:where+1] and [where+1:].