Add occur mode to globally ignored projectile modes.

This is just a suggestion (there might be better ways of doing this):

When using multi-occur (C-c p o) multiple times with the same regex, it lists
all the matches in the opened occur buffers. Adding the occur-mode to the
ignored modes stops this.

As this is a customizable variable, it's easy for each user to achieve this, but
I think the default behavior should be to not show matches in other/already open
occur buffers.