Add support for XPM. (#3706)

* .xpm and .pm extensions associated with XPM.

* .pm is disambiguated by searching the /* XPM */ string.
  This is how `file` performs detection and should work with
  every XPM3 file (most XPM generated by software later than 1991).

Added XPM samples:

* stick-unfocus.xpm: extracted from Fluxbox (MIT License)
  0c13ddc0c8/data/styles/Emerge/pixmaps/stick-unfocus.xpm

* cc-public_domain_mark_white.pm: public domain image from
  https://commons.wikimedia.org/wiki/File:Cc-public_domain_mark_white.svg
  converted to XPM with ImageMagick (convert input.svg output.xpm).

test/test_language.rb

@@ -63,6 +63,7 @@ class TestLanguage < Minitest::Test
     assert_equal Language['Vim script'], Language.find_by_alias('vim')
     assert_equal Language['Vim script'], Language.find_by_alias('viml')
     assert_equal Language['reStructuredText'], Language.find_by_alias('rst')
+    assert_equal Language['XPM'], Language.find_by_alias('xpm')
     assert_equal Language['YAML'], Language.find_by_alias('yml')
     assert_nil Language.find_by_alias(nil)
   end