Allow a formatter rule to override the executable name

[knu]

This introduces a keyword symbol :executable to format-all-formatters.

It solves a typical problem where the predefined formatter is just fine but the executable you want to use instead is installed under the project working directory or elsewhere not necessarily in the default PATH.

e.g.

(setq format-all-formatters
      '(("JavaScript"
         (prettier
          :executable
          "node_modules/.bin/prettier"))))