Add support for custom text alignment

[GeertJohan]

Fixes #69

Example usage:

foo = Html(
...
  customTextAlign: (dom.Node node) {
    if (node is dom.Element) {
      switch (node.localName) {
        case "p":
          return TextAlign.justify;
      }
    }
  },
...
);

[Sub6Resources]

Thanks for the pull request! At first glance this looks like it will work fine. I'll complete the actual review later today and work on merging and releasing within the next couple days.

[GeertJohan]

Hi @Sub6Resources, is there anything else I can do to get this merged? Thanks!

[Sub6Resources]

Nope, I've got it covered. Thanks though! The release should be available on Pub within the next couple hours.

[GeertJohan]

Thanks @Sub6Resources !