Add g:vim_man_man_cmd to change man command (taken from blp1526)

[benpjohnson]

Hi,

I've taken this change from @blp1526 #24 which is very useful and added the extra changes requested on the original pull request.

Thanks,

Ben

[bruno-]

Thanks! I wanted to change a couple things before merging. That's now done so this is in. You're still the committer though.

Changes:

• changed variable name to g:vim_man_cmd (an extra _man would be correct, but it looked weird)
• squashed commits
• documented this option in the docs

[benpjohnson]

Thanks bruno all credit goes to @blp1526 anyway!