ability to edit bindings

[craftyguy]

This would be very useful to have! For example, I might set something like :tab rooms to C-o, overriding the default. Or I might add a new binding to run some other command. It might be fun if it supported chaining multiple commands together (a silly example might be a binding to something like :tab rooms<cr>/foobar<cr>iheeeey<cr>)

[ptman]

#150