Escape subshell commands

[b-ryan]

Maybe a viable fix for #264 ?

I couldn't find any workable solution using existing python code. Even shlex.quote didn't seem to work from my testing. Instead, this branch escapes backticks and the dollar sign.

Tests:

Some thoughts:

• This is only appropriate for bash & maybe zsh. I'm also not sure it's a sufficient change.

[b-ryan]

I did some basic testing and this problem doesn't appear to affect either fish or zsh. So I modified my code to only apply itself to bash shells. Testing is going well. Going to merge this since it's better than letting the bug sit out there.