Improve the description of the -reporter option.

This documents that you can add multiple -reporter options to
add more reporters, and that the pretty and user-notifications
reporters only are added if you don't use -reporter on the command line.

This also documents that the user-notifications reporter won't be used
if Travis CI is detected.
Merge pull request #449 from tipbit/readme-reporters-improvements

Improve the description of the -reporter option.