Modifiable mode line

I tend to work on a smaller screen, with buffers split vertically. This leaves me little space for long strings describing mode names. Generally I use [diminish-mode](http://marmalade-repo.org/packages/diminish) to reduce string names, but this isn't the best option for projectile because I find the dynamically changing string showing the name of the project useful.  

By making `projectile-mode-line-lighter` a customizable variable, people can alter the prefixed "Projectile" as desired. I've moved the leading space out of the variable so that altering the variable doesn't require knowledge that the leading space is necessary for the mode lines to be separate, but it's questionable whether or not that's the right way to do it and I would be happy to change that if you think it's a better idea to leave the leading space under user control.