Enable defining compilation/test commands in file-/dir-local vars.

They should be safe as long as they're strings.
Add helm to allowable completion systems
Merge pull request #371 from immerrr/mark-cmds-safe-if-strings

Enable defining compilation/test commands in dir-local vars
Check whether helm completion function is defined
Merge pull request #378 from alanpearce/helm

Add helm to allowable completion systems
Update projectile.el
Merge pull request #380 from peterhoeg/patch-1

Update projectile.el