Blinking cursor support for iterm, konsole.

This pull request adds 3 new options for togglecursor shapes. (blinking_{block,line,underline})
1) Since iTerm has supported DECSCUSR since 2012, this plugin now uses xterm ctrl seqs when used within iterm.
2) Blinking support for konsole using one of its proprietary escape sequence.
3) VTE based terminals(gnome-terminal etc) will support DECSCUSR soon(If you're impatient, you can grab the patch here: https://bugzilla.gnome.org/show_bug.cgi?id=720821). After it does, we can uncomment line 41 to support all VTE based terminals!