Add boolean version of setAttr

[ryukoposting]

Related issue: #239

This PR adds a new setAttr proc, which accepts a boolean value instead of a string. When the value is true, it's the same thing as doing vnode.setAttr("key", val = ""). When the value is false, it iterates through VNode.attrs and deletes the attribute, if it exists.

The net effect of this new proc is that it is much easier to deal with attributes that need to be dynamically added/removed from the VDOM.

Example:

proc submitButton(dataIsValid: bool): VNode =
  buildHtml(tdiv):
    button(disabled = not dataIsValid):
      if dataIsValid:
        text "Submit"
      else:
        text "Cannot submit, data is invalid!"

[geotre]

@ryukoposting did you have any luck with the methods described in #239? My concern with this approach is that it will be slower as it must iterate through the attributes, whereas the existing toChecked/toDisabled/toSelected do not need to. What do you think?

[ryukoposting]

@ryukoposting did you have any luck with the methods described in #239? My concern with this approach is that it will be slower as it must iterate through the attributes, whereas the existing toChecked/toDisabled/toSelected do not need to. What do you think?

Apologies for the delay- got busy with work/personal things again. The performance hit associated with iterating through a very short list is miniscule compared to the overhead that comes with every VDOM re-draw. If you can perceive a difference, then sure. I like my solution because it is more generalized- i can set any kind of boolean attribute, whether it's selected, contenteditable, or notdefinedbythedombutcouldbeusedforsomething.

[geotre]

After some more thought, I think this will work well if we also use the same method as the existing helpers (when on js backend). See suggested change above @ryukoposting

Changes implemented

[geotre]

This looks great now @ryukoposting, good job adding documentation also.

@Araq I think this is ready to go if you want to merge

[geotre]

Ok I'm going to go ahead and merge this so I can create a new release. Araq let me know if you want me to handle things differently in the future