Add diff task

[xafizoff]

I want to run coverage analysis only for lines that have been changed. A new coveralls.diff task checks coverage on lines that were added up to HEAD from git revision specified in the --from-git-rev option. If total coverage for those lines is less than minimum (--threshold option), then task exits with status 1.

Task will:

• Print total coverage information for changed lines.
• Print total coverage information for changed lines for each file.
• Print changed lines and highlight covered and missed lines.

$ MIX_ENV=test mix coveralls.diff --from-git-rev master