Add support for boolean attributes

[nmattia]

In the following HTML

<div disabled></div>

disabled is a boolean attribute.

2.5.2 Boolean attributes
A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.
If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.
The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

reflex-jsx doesn't support boolean attributes. Reflex doesn't either out of the box, but boolean attributes could be replaced with attr="attr". This should only require changes to the parser.