Add variable to disable lisp-indent-offset sometimes

[jgkamat]

Sorry for the delay, I actually completely forgot about this...

I actually changed the spec of the variable from what I originally defined it as, the docstring is now:

Selectively ignore the value of indent_sizefor Lisp files.
Prevents `lisp-indent-offset' from being set selectively.

nil - `lisp-indent-offset' is always set normally.
t   - `lisp-indent-offset' is never set normally
       (always use default indent for lisps).
number - `lisp-indent-offset' is not set only if indent_size is
         equal to this number.  For example, if this is set to 2,
         `lisp-indent-offset'will not be set only if indent_size is 2.

Let me know if anything seems wrong here, since this is a pretty large change.

Closes #137

[jgkamat]

Is there a better place this can go? I don't want it to go in the submodule, it seems specific to this plugin.

[10sr]

Good! Thanks 🙌