Merge lib/compiler/passes.js into lib/compiler.js

It didn't make sense to have the passes in a separate file.
Plugin API: Implement the |passes| parameter for |PEG.compiler.compile|

The |passes| parameter will allow to pass the list of passes from
|PEG.buildParser|. This will be used by plugins. The old way via setting
the |appliedPassNames| property is removed.

Implements part of GH-106.
Plugin API: Implement the |plugins| option for |PEG.buildParser|

The |plugins| option allows users to use plugins that change how PEG.js
operates.

A plugin is any JavaScript object with a |use| method. After the user
calls |PEG.buildParser|, this method is called for each plugin with the
following two parameters:

  * PEG.js config that describes used grammar parser and compiler
    passes used to generate the parser

  * options passed by user to |PEG.buildParser|

The plugin is expected to change the config as needed, possibly based on
the options passed by user. It can e.g. change the used grammar parser,
change the compiler passes (including adding its own), etc. This way it
can extend PEG.js in a flexible way.

Implements part of GH-106.
Plugin API: Split compiler passes into stages

The compiler passes are now split into three stages:

  * check -- passes that check for various error conditions

  * transform -- passes that transform the AST (e.g. to perform
    optimizations)

  * generate -- passes that are related to code generation

Splitting the passes into stages is important for plugins. For example,
if a plugin wants to add a new optimization pass, it can add it at the
end of the "transform" stage without any knowledge about other passes it
contains. Similarly, if it wants to generate something else than the
default code generator does from the AST, it can just replace all passes
in the "generate" stage by its own one(s).

More generally, the stages make it possible to write plugins that do not
depend on names and actions of specific passes (which I consider
internal and subject of change), just on the definition of stages (which
I consider a public API with to which semver rules apply).

Implements part of GH-106.
Plugin API: Implement the --plugin option

Implements part of GH-106.