Add helm-projectile-find-other-file

Add proper Helm command for projectile-find-other-file. It behaves
exactly like projectile-find-other-file but has Helm actions and able to
open file in other window as well as open multiple files.

This command has its own function because C-u is used for flex-matching
instead of invalidating cache.

Also refactor helm-projectile-command macro to accept prompt string.