Allow SendSelection to use marks and ranges

[bawalsh3000]

In situations where you are making tweaks and repeatedly sending the same section of code, it would be helpful if you could just use marks and send a command like: 'a,'bSlimuxREPLSendSelection

[dvbuntu]

You can definitely send ranges since 318e06da8. I never actually tried it with marks.

[hsanson]

Using vim-topo plugin this is easy to accomplish. See this comment for more details.