Add clear parameter to the BR macro

[Christian Boos]

I had to write [[html(<br style="clear: left"/>)]] once too many…

Such HTML snippets are very useful at times but are a bit painful to write, not to mention it feels like a hack…

Being able to use the BR "macro" for that feels much more natural:

[[br(clear:left)]]

or:

[[br(clear=left)]]

[[br(clear)]] alone would stand for [[br(clear:both)]].

I think [[br]] is probably not much used since the introduction of the WikiCreole equivalent \\, but as it's still there in the code and documented in WikiFormatting, why not making it more useful?

See log:cboos.git@t13117-wikimacro-BR-clear.