allow use of backrefs in lineinfile

allow the use of backreferences in the replacement. useful for commenting lines in and out. removed the check regexp == line to make this work. here are some examples why i think it's useful:

commenting out 'splashimage=' in grub.conf:

lineinfile dest=grub.conf state=present regexp='^(splashimage=.*)$' line="#\\1"

commenting the line in:

lineinfile dest=grub.conf state=present regexp='^#(splashimage=.*)$' line="\\1"

backreferences allow also some pretty nice things like changing only a part of a line, e.g. change splash image. note, the splash image option contains host specific parts like '(hd0,0)':

lineinfile dest=grub.conf state=present regexp='^(splashimage=.*/)splash.xpm.gz$' line="\\1mysplash.xpm.gz"