Support boolean values for boolean attributes

[schneiderfelipe]

Fix #20. Implemented as suggested in comment:

julia> using Hyperscript

julia> m("input", type="checkbox", checked=true)
<input checked="" type="checkbox" />

julia> m("input", type="checkbox", checked=false)
<input type="checkbox" />

julia> m("input", type="text", value=true)  # as string if not a boolean attribute
<input value="true" type="text" />

julia> m("input", type="checkbox", checked="true")  # boolean attributes expect booleans
<inputError showing value of type Hyperscript.Node{Hyperscript.HTMLSVG}:
ERROR: Boolean attribute "checked" expects `Bool`, found `String`: <input> checked=true  />
Stacktrace:
  ...