add options for composer and php binaries

[koenpunt]

Added :composer_bin and :composer_php options for better control of which binaries to use. When not explicitly set, the existence of the composer and php executables is now validated before the tasks are run.

Also Composer is now always invoked through php. For this the path to the composer executable is expanded by calling which composer on the remote host.

When :composer_bin is set to :local, composer:install_executable is automatically invoked on deployment. So no more manual hooking of additional tasks in the deploy configuration.

This more or less supersedes #41, but I think this is more flexible.

[koenpunt]

ping ✨

[ndench]

This PR would be very helpful since I'm using composer installed as a .phar, so it's not in the default location. Would be great to be able to specify the location of composer.

[lucasmezencio]

Up!

[dhyegocalota]

👍