Mark with RMB+LMB mouse command (workaround for mark on drag select bugs)

[mg979]

My take for solving this issue, that for me it renders the plugin unusable (it also breaks find_under_expand and just too much in my installation). Moreover, it is just a mouse thing, because selecting with keyboard doesn't mess up anything.

With this change, to mark with mouse you have to press right button + left button. It doesn't replace normal function (expand to line/paragraph/all), it just precedes them, they're still possible with one more click.

An advantage over the previous implementation is that you don't need to select anything, you can just set the mark at the cursor if that's what you want.

Another advantage is that, being a command, you can set the mouse binding you want (to extra mouse buttons if you have them, for example).