Add option to disable bar-style fake cursors

[smithzvk]

I use bar style cursors and have had issues with the fake cursor rendered in a "bar style". I have been disabling this feature for years, but figured it might be nice to have an official customization to do so.

[smithzvk]

This failure seems unrelated to this change. A Emacs-24 builds broken for some other reason right now?

[smithzvk]

I see that there is discussion about this issue on the original PR #253 introducing this behavior and there is a live PR which is attempting to fix a related issue in a different way, #366. The suggested solution from the original PR author, @jtamagnan, is to clobber the mc/cursor-is-bar function with something like:

(defun mc/cursor-is-bar () nil)

...after load. I'm switching to that for know as I'm tired of remembering to patch this library every time there is an update.

I think this whole "render fake cursors as bars" functionality is causing more trouble than it is worth, but the clobber method is a viable solution for me if we're all too busy to consider this PR or don't want this change for whatever reason.

I'll leave it open, feel free to close as you see fit.

[jtamagnan]

Sorry this took so long to look at! Looks good to me! Thank you for your patience and the contribution!