add HTTP language

Useful for `curl -i` dumps. Had to add primary_extension although this
data is usually not saved in files, but shown as code blocks.

lib/linguist/languages.yml

@@ -579,6 +579,10 @@ HTML+PHP:
   extensions:
   - .phtml
 
+HTTP:
+  type: data
+  primary_extension: .http
+
 HaXe:
   type: programming
   lexer: haXe