js-yaml doesn't always provide line/mark data for certain exceptions;
handle this scenario more gracefully

Closes #4600

Closes #4308

Closes #4025

Fix "Uncaught TypeError: Cannot read property 'severity' of undefined"

When hovering a line with multiple lint annotations, codeMirror will
now show all the attached markers instead of just the first one

Closes #3538

Issue #3423

... and export (via defineExtension) a method to explicitly trigger
linting from user code.

This makes it impossible to change the mode of the editor
and still automatically get appropriate annotations.

Issue #3227

`options` with a `globals` key as defined in a standard `.jshintrc` is actually separated into two arguments to the `JSHINT` function in the `jshint` cli: `https://github.com/gameclosure/jshint/blob/master/src/cli.js#L487`.  This forwards `options.globals` properly to `JSHINT`.

Issue #3104

So that the helper mechanism can meaningfully be used to register
asynchronous linters.

Use host property, not root.

Issue #3039

Since the mouseover will necessarily happen at the very edge of the node, and thus
give ambiguous coordinates.

Closes #2821

This might break sites that depend on async linters getting an editor instance
as first arg, or sync linters getting the .options property of the options passed
as second argument. The args passed are now more or less consistent.

Issue #2681

Bumping them for every release just adds too much git
noise and history blowup.

Closes #2376

Added a wrapper for js-yaml.js to enable YAML lint support. Followed
json-lint.js pattern. Unfortunately js-yaml.js only shows one error at
a time, but something is better nothing. Checked code with linter and
ran tests. All came back clean.

Passing options causes issues in jshint since lint options are also
passed. These options cause bad option errors in the annotation list.

e.g. errors on css `input.someLongClassName {}`

Original error & warning icons had jagged edges due to being GIFs
This made them look quite poor/low quality on dark BG'd themes
So, new, better looking icons added which have been run thru TinyPNG for
compression
Result is fewer bytes than original and they look good on dark BG's too

Issue #1782