Don't apply extension automatically, makes it impossible to keep
existing extension without issue (see
https://github.com/gruntjs/grunt-contrib-compress/issues/79) if the user
wishes to add an extension they can do so via ext or rename.

Closes GH-91
0.10.0
Merge remote-tracking branch 'origin/master' into gzip_min_length

Conflicts:
	README.md
	tasks/lib/compress.js