feat(bind): support options in @sessionx-bind

[znd4]

I just learned from cmp-git that #7 already exists. I suppose this is an alternative solution for #85.

An example of using this would be

set -g @sessionx-bind '-n M-o'

I'll add an example to the README, and no hard feelings if you prefer #7

security concern?

I call out in the code comment that this is kind of intentionally having users inject code in a string input, but I can't imagine why anyone would put untrusted input here , so... 🤷 ?