First cut of JSX-like support

[myitcv]

A runtime stop-gap solution for JSX-like support (will ultimately be replaced with a proper compile-time solution), per #64.

Also includes a first-cut of reactVet to catch misuse of the new myitcv.io/react/jsx package.

Allows things like:

func (a *MyCompDef) Render() r.Element {
	return r.Div(nil,
		jsx.HTML(`
		<h1>Hello world</h1>

		<p>This is my first GopherJS React App</p>
		`)...,
	)
}

or via Markdown:

func (a *MyCompDef) Render() r.Element {
	return r.Div(nil,
		jsx.Markdown(`
# Hello World

This is my first GopherJS React App
		`)...,
	)
}

Godocs:

package jsx
    import "myitcv.io/react/jsx"


FUNCTIONS

func HTML(s string) []r.Element
    HTML is a runtime JSX-like parser. It parses the supplied HTML string
    into myitcv.io/react element values. It exists as a stop-gap runtime
    solution to full JSX-like support within the GopherJS compiler. It
    should only be used where the argument is a compile-time constant string
    (TODO enforce this within reactVet). HTML will panic in case s cannot be
    parsed as a valid HTML fragment

func HTMLElem(s string) r.Element
    HTMLElem is a convenience wrapper around HTML where only a single root
    element is expected. HTMLElem will panic if more than one HTML element
    results

func Markdown(s string) []r.Element
    Markdown is a runtime JSX-like parser for markdown. It parses the
    supplied markdown string into an HTML string and then hands off to the
    HTML function. Like the HTML function, it exists as a stop-gap runtime
    solution to full JSX-like support within the GopherJS compiler. It
    should only be used where the argument is a compile-time constant string
    (TODO enforce this within reactVet). Markdown will panic in case the
    markdown string s results in an invalid HTML string