userdiff: add support for Fountain documents

Add support for Fountain, a plain text screenplay format.  Git
facilitates not just programming specifically, but creative writing
in general, so it makes sense to also support other plain text
documents besides source code.

In the structure of a screenplay specifically, scenes are roughly
analogous to functions, in the sense that it makes your job easier
if you can see which ones were changed in a given range of patches.

More information about the Fountain format can be found on its
official website, at http://fountain.io .

Signed-off-by: Zoë Blade <zoe@bytenoise.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -527,6 +527,8 @@ patterns are available:
 
 - `fortran` suitable for source code in the Fortran language.
 
+- `fountain` suitable for Fountain documents.
+
 - `html` suitable for HTML/XHTML documents.
 
 - `java` suitable for source code in the Java language.

t/t4018-diff-funcname.sh

@@ -31,6 +31,7 @@ diffpatterns="
 	cpp
 	csharp
 	fortran
+	fountain
 	html
 	java
 	matlab

t/t4018/fountain-scene

@@ -0,0 +1,4 @@
+EXT. STREET RIGHT OUTSIDE - DAY
+
+CHARACTER
+You didn't say the magic phrase, "ChangeMe".

userdiff.c

@@ -35,6 +35,8 @@ IPATTERN("fortran",
 	  * they would have been matched above as a variable anyway. */
 	 "|[-+]?[0-9.]+([AaIiDdEeFfLlTtXx][Ss]?[-+]?[0-9.]*)?(_[a-zA-Z0-9][a-zA-Z0-9_]*)?"
 	 "|//|\\*\\*|::|[/<>=]="),
+IPATTERN("fountain", "^((\\.[^.]|(int|ext|est|int\\.?/ext|i/e)[. ]).*)$",
+	 "[^ \t-]+"),
 PATTERNS("html", "^[ \t]*(<[Hh][1-6][ \t].*>.*)$",
 	 "[^<>= \t]+"),
 PATTERNS("java",