autoload.lua: add function and keybind to call script manually

[dieEisdiele]

• New function enables current instance of the script then calls
  find_and_add_entries()
• Keybind has no default binding and calls the new function

Allows users who only want to run the autoload script occasionally to
set disabled=yes then call the script during playback by binding a
shortcut in input.conf, e.g.:
ctrl+p script-binding autoload/enable ; show-text "Playlist loaded"
Without this change, find_and_add_entries() is only called at the
start of a file. This means that it is still possible to enable the
script during playback, but it requires forcing the current file to
restart, e.g.:
ctrl+p change-list script-opts append autoload-disabled=no ; playlist-play-index current

This method is more seamless and should not affect users who do not
want this functionality, as it does not add any events which will cause
find_and_add_entries() to be called other than the keybind, which is
unbound by default.

[dieEisdiele]

Sorry for making the same pull request repeatedly! It's my first time making pull requests and I wasn't sure how to fix the capital letter in the commit title setting off a linting error.

[Dudemanguy]

You don't need to close and open new PRs. You can just force push to the same branch and github will update it.