Add functions that checks for conflicting zsh options and inverts them (also fixes #2)

This PR basically consist on two parts:

+ A collection of three functions that allows:
    + Check if zsh is running zsh
    + Invert a conflicting option
    + Re-enable the previous (if any) disabled options
+ Disabling and re-enabling the `noclobber` option (in zsh) that makes the `draft`
  and `revise` to fail

I tried hard to make this PR as POSIX compliant as possible, but I'm not sure I fulfilled that objective.

Oh well, if you think needs improvement (no doubt it will), please tell me.