Fix #79 - add Remove-MergedGitBranch command

[rkeithhill]

Not sure if this command should require confirmation by default.

Perhaps we also need a Remove-GitBranch command which would need
confirmation.

Fix #79

[rkeithhill]

Also, if using --merged I don't think you'd ever need to use -D (force delete), would you?

[rkeithhill]

Also, wondering if this should be just Remove-GitBranch and then add a -Merged switch to change its behavior to remove only merged branches. Or we could default to only deleting merged and add a -All switch to delete all branches except the default exclude pattern (excludes the current branch as well as master and develop).