Add CLI flag '--diff-frontend' to set diff display.

[0x6d6e647a]

• Support for all merge frontends as diff frontends.
• Add support for colordiff and diff as diff frontends.
• Add support for kompare as a merge and diff frontend.
• Add synonym flag '--merge-frontend' for '--frontend'.

Fixes xsuchy/rpmconf#48

[0x6d6e647a]

Force push to my branch was to fix the commit email address. Sorry for any confusion.