Add indeterminate attribute for checkboxes

[dyerc]

I couldn't see any other way of setting a checkbox to an indeterminate state, so I have effectively mirrored the way a checked state is set. It seems to work fine when tested with my app.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

Can you provide MDN or W3C docs about this?

[dyerc]

It is mentioned here: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input

A check box. You must use the value attribute to define the value submitted by this item. Use the checked attribute to indicate whether this item is selected. You can also use the indeterminate attribute (which can only be set programmatically) to indicate that the checkbox is in an indeterminate state (on most platforms, this draws a horizontal line across the checkbox).

Is the way I implemented this the best way of setting the value 'programmatically'?

[evancz]

Not sure. Have you tested that your implementation works? Sometimes you need to use attribute instead of property to get things working on all browsers.

I'd like to add this to #53 which is tracking a bunch of changes so we can make a more coherent release.

[dyerc]

I have just given this a test in Chrome, Safari and Firefox on Mac, and IE 11 and Chrome on Windows. It seems to work fine in all of them. Chrome and all Mac versions show a horizontal line through the box, and IE shows a smaller square within it.

[evancz]

Seems reasonable. Added to #53 so I can review all these changes in one big batch and get them released together.