Rework the linting support to support disabling eslint errors.

[lddubeau]

We also gain support for adding the @ts-ignore flag that the tsc compiler understands.

This change adds logic for identifying the general type of checker from which flycheck errors are reported, and so be able to add the right flag when the user requests that an error be turned off.

This tackles point 2 of this post.

I've been running this for a while in projects that use eslint for all linting work, and projects that still use tslint, and it's been working for me.

[josteink]

Sorry about the late response.

If it's OK, I'll hold on investigating this PR, until the questions I've raised in #308 are put down or resolved :smile: