Add pattern for matching filenames to the Liquid lexer (#1351)

The Shopify Liquid parser and the Jekyll Liquid parser treat the
argument to the `include` tag differently. Specifically, the Jekyll
lexer expects a filepath.

The existing Liquid lexer does not properly handle code written for the
Jekyll implementation. This commit fixes that by introducing a rule thet
matches against slashes, periods and alphanumeric characters.

lib/rouge/lexers/liquid.rb

@@ -266,7 +266,7 @@ class Liquid < RegexLexer
         end
 
         rule %r/(with|for)\b/, Name::Tag
-        rule %r/[^\s\.]+(\.[^\s\.]+)+\b/, Name::Other
+        rule %r/[\/\w-]+(\.[\w-]+)+\b/, Text
 
         mixin :variable_tag_markup
       end

spec/visual/samples/liquid

@@ -57,8 +57,8 @@ Some other {{ output }}!
 {% else %}
 {% endcase %}
 
-{% include file.html param = 'value' param2 = object %}
-{% include 'snippet', param: 'value', param2: object %}
+{% include dir/file.html param = 'example.com' param2 = object %}
+{% include 'snippet', param: 'example.com', param2: object %}
 {% include product_page with products[0] %}
 {% include {{product_page | split: "." | first}} for products %}