Only fail on error, add option to ignore warnings

[Raynes]

This pull request extends on the error checking a bit to ignore
warnings. Warnings do not necessitate a complete failure. For example,
with ES6, you're gonna get warnings about new stuff no matter what –
you can still generate docs!

Furthermore, added an ignoreWarnings option to not print warnings.

[krampstudio]

Isn't the role of the force flag to ignore warnings ?

[Raynes]

Yes, but the way this is being done makes no sense. It's failing immediately on warnings. Warnings in jsdoc and warnings in grunt are two different things. Most importantly, even if you --force, you don't see all the warnings and errors. jsdoc doesn't even die during errors, it keeps going and shows you all of them. I mostly just want all of the output, warnings included.

[simonomis]

This feature would be really useful for me too. I'm writing ES6 modules and because I have to use force I'm missing genuine jsdoc errors because the exit status is always 0. Being able to ignore warnings (because they are almost always about ES6 import/export nodes) but still fail on errors would be very helpful at least until all ES6 syntax is fully supported.

[krampstudio]

I'll merge it as soon as I have some time to do it.

[krampstudio]

Merged but I need to handle the ignoreWarning option and do some testing before publishing to npm.