Allow surround plugin customisation.

Adds :surround <char> <definition> command to define new or redefine
existing replacements.
<definition> must not contain spaces but '<SPACE>' can be used if needed.
The surround point is specified by '\r':
:surround \ /*<SPACE>\r<SPACE>*/
:surround / /*\r*/
Merge pull request #217 from igrekster/master

Surround customization command.