Added variable for name of phpcs executable.

Also renamed `flymake-phpcs-command' to
`flymake-phpcs-build-command-line' so the new variable can use the name
`flymake-phpcs-command' (better compatibility with
https://github.com/illusori/emacs-flymake-phpcs).
Merge pull request #2 from arnested/phpcs-commmand

Added variable for name of phpcs executable.