Add .theme to .gitattributes

Problem/Motivation

Follow-up to #1111224: Introduce .gitattributes to end the CRLF/LF and binary diff horror, and detect/auto-fix whitespace errors

The .gitattribues files specifies the file type for certain extensions (like *.module is a PHP file) so that the hunk headers are more useful. From http://www.kernel.org/pub/software/scm/git/docs/gitattributes.html

diff

The attribute diff affects how git generates diffs for particular files. It can tell git whether to generate a textual patch for the path or to treat the path as a binary file. It can also affect what line is shown on the hunk header @@ -k,l +n,m @@ line, tell git to use an external command to generate the diff, or ask git to convert binary files to a text format before generating the diff.

However, we are missing the .theme file from this list, so we aren't getting less-awesome patches against a theme's PHP code.

Proposed resolution

1 line change to .gitattributes to add .theme.

User interface changes

None.

API changes

None.