Allow non-boolean field flags

[davidism]

I want to support more HTML validation attributes, but currently the flags only set their value to True. For things like maxlength, we need to set the actual value.

My idea is to allow field_flags to be a function which returns a dict, deprecating and eventually removing the old tuple behavior.

[whb07]

This sounds great! A lot of the HTML5 standards can help reduce the minutiae of server side validations.

[Sohalt]

Any progress on this? Would you accept a pull request?

[davidism]

I'd be happy to review a pull request. Just take that as a given for any open issue that is not under discussion and doesn't have a PR already, no need to ask. 😄

[Sohalt]

I've opened a pull request. The stylecheck is failing however, because some lines are too long. I'd like to have your opinion on that, since the contributing guidelines state, that there is no hard limit on line length.