Separate dev / production linting

[glebec]

Pursuant to a conversation between @joedotjs and myself, the no-debugger rule — currently set to level 2 (error) to prevent committing a debugger statement — causes the lint task to fail, in turn stopping the build.

A solution would be to separate dev linting from production linting. The former could have no-debugger set to level 1 (warning), letting students know it should be removed prior to commit; the latter could have no-debugger set to 2 (error), providing an early out for build.

To accomplish this, gulp-eslint can take an options object using the same CLI options as specified here. The configFile option will let us programmatically select which eslintrc to apply. Alternatively — and perhaps preferably? — the rules option can let us add production-specific rules to override the base config.

There may also be other alternatives; research accordingly.