src: don’t require a mode
src: don’t require a mode

A better pattern looks something like this:

  var editor = new Compose(element)
  ...
  editor.use(RichMode) // Or whatever

Not requiring that a mode be specified at insantiation has several
benefits:
  - The setup phase is done by the mode, so this change allows plugins
    to be added before initialization; closes #75.
  - Offering a shorthand for choosing between modes means Compose needs
    to be bundled with whatever mode(s) it provides a shorthand for.
    This increases the overall size of the browserified package for
    little gain; most consumers will probably use only one of the
    bundled modes, not all.
  - It increases modularity. One can choose to have a multi-paragraph
    mode, and optionally add a rich text plugin beforehand if needed.

lucthev/compose,lucthev/compose