Add tag() class for create custom elements

[cpsievert]

Closes #148

[coveralls]

Coverage decreased (-0.1%) to 97.922% when pulling ee5cd72 on cpsievert:custom-tags into 479e4ea on Knio:master.

[songololo]

This would be useful, any chance of merging?

[Knio]

Thanks, but sorry, I'm not really interested in this. This is very trivial for anyone to write for themselves, not something that most users will use, and I want to keep this library very minimalistic.

Here's another slightly different version which seems shorter and nicer to me (assuming you re-use the same custom element in multiple places):

class custom(dominate.tags.html_tag): pass

custom(foo=1).render()
'<custom foo="1"></custom>'

I'd be slightly more willing to take a parameter to dom_tag, maybe a __tagname similar to __inline etc. for the non-reuse case.

[songololo]

Thanks @Knio

Can this example be added to the README?

[Knio]

@songololo Sure, but maybe the wiki would be a better place for patterns like this? The readme is fairly long already.

[songololo]

@Knio yes, a wiki page with common usage examples for these sorts of cases would be really handy.