[WIP] Support for multiple paths

[ezzatron]

This PR adds support for specifying multiple paths as command line arguments to the peridot executable.

Summary/justification of changes:

• This is a BC break, because previously plugins could define additional CLI arguments, and having an array argument makes that impossible.
  • The docs mention this for the peridot.start event: "Useful for defining additional CLI arguments and options.", and will need to be updated.
  • Plugins will still be able to define CLI options.
  • Because of this InputDefinition::argument() is gone.
  • Perhaps if really necessary we could allow prefixed arguments? Probably not worth it.
• Retains support for getPath() and setPath().
  • We can deprecate them with the next major release, and remove them in the one after that. I'm not sure how we want to handle deprecations though.
• I'm not sure what the PERIDOT_PATH environment variable is used for, but I also retained support for it, and added a similar PERIDOT_PATHS environment variable, which contains all the paths, separated by PATH_SEPARATOR.
• Added a default set of paths (spec, specs, test, tests).
  • This helps simplify the common case of running specs from a project's root.
  • It also gets rid of the need to specify this path in peridot.php config, and less config = more better.
  • If a path is explicitly specified, it is an error for it to not exist, but default paths are allowed to not exist. This matches the behavior of PHPUnit and Mocha. EDIT: This unfortunately requires reflection h4x, because of a limited Symfony Console interface.
• Another BC issue is setting the default path in peridot.php like this.
  • This can still be done, but the default value needs to be an array.
  • peridot-jumpstart should be updated to remove this line, since there will be default paths out of the box.
• I had to shuffle around some input definition stuff to fix an issue with using peridot --help.
• Symfony's built-in CLI options seem to be bugged, despite appearing in --help output. I'm not sure if this is an existing problem or something I've introduced. EDIT: Fixed.

[codecov-io]

Current coverage is 96.33% (diff: 76.74%)

Merging #200 into master will decrease coverage by 0.81%

@@             master       #200   diff @@
==========================================
  Files            21         21          
  Lines           736        763    +27   
  Methods         180        184     +4   
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            715        735    +20   
- Misses           21         28     +7   
  Partials          0          0          

Powered by Codecov. Last update 25ec1ce...d53b51f