upgrade eslint and use the higher level CLIEngine interface

[julienw]

The current gobble-eslint does not work for me:

• it tries to find .eslintrc but I use .eslintrc.json (which is recommended nowadays)
• it does not use eslint 2
• the output does not use the nice current formatter

So I redid it, using the higher level CLIEngine API that is likely more future proof as well.
But I removed the functionality about the sourcemap because I don't use it (I prefer to lint my source files than a transpiled file). So tell me what you think.
Do you think I should create a gobble-eslint-ng instead ?

[julienw]

as a side note, I wonder if linters should not be implemented as transforms instead of observers: I think they should transform into "nothing" so that we can use them as part of the normal build without being merged. What do you think ?

[julienw]

Mmm this seems to work properly only as a first step, not after any other node. I'll debug this later.

[julienw]

I added "options.dotfiles = true" to tell eslint that it's OK if our files are in .gobble, and I filed eslint/eslint#5427 at eslint.