Map commands to sublime command

[math2001]

It would be nice to be a able to map a command in Six's command mode to a Sublime command

This would solve #236, and also be nice for linters for exampe (so we can "implement" cn and cp)

[guillermooo]

This is planned, and probably similar to plugins. I don't have a specific timeline for it, but I'm close to wrapping up porting Six.Current code to Six.Next (in fact, mostly just plugins and mappings are missing now). After that's done, I can start looking at this.