Replace "compiler" with "make"

The "compiler" option was a poor fit for several reasons:

* It restricts to the paltry set of built-in and installed compiler
  plugins.
* It doesn't allow arguments.
* It forces third party editor implementations (if there ever are any)
  to be aware of Vim compiler plugins.
* Its name suggests using it not for a build tool but an actual
  compiler, a usage better suited to "dispatch".
* Rails.vim's implementation has a conflicting usage.

Instead, let's provide a method to set 'makeprg', and try to
heuristically derive the compiler plugin from that.  The initial
heuristic is "executable name".  It would be ideal (but likely
infeasible) to support more elaborate constructions such as

    ruby -C {project} -S bundle exec rake