Allow customization of the initial frame name.

Rather than hardcode the use of "main" as the initial frame's name, use `persp-initial-frame-name` to allow a user defined string.
(perp-initial-frame-name): Fixed Docstring
(persp-show-modestring): Change to defcustom

Set as choice to allow choice between off (nil), modeline (t) and header ('header).
Merge pull request #20 from jleechpe/master

Allow customization of the initial frame name.