add command line option to specify a vim command

Adds the command line option -c to vimcat and vimpager, so that we can
run a vim command after opening the file. For instance

  cat blah | vimpager -c 'set ft=sh'

opens blah highlighted as a shell script.
document '-c' command line argument
Merge pull request #70 from aroig/clargs

add a command line argument to run a vim command after opening the file