don't require .git postfix on remote addresses

Modified-by: Robin Smidsrød <robin@smidsrod.no>
added `prq checkout` feature

Added a feature to automate the process of creating a new remote
corresponding to the contributing repository of a pull request and
checking out the commit referenced by its branch HEAD.

Checks should be added for pre-existing remotes.

Documentation still needs to be updated.
don't try to create a new remote when it already exists
Added checkout command
Fixed a bug with reading exit code from external programs
Updated changelog
Bumped version number
v0.2.0

    - Added checkout command (PR#4)
    - Fixed a bug where github URLs without .git extension
      weren't recognized (PR#3)
    - Made error checking when running external programs more robust