support binding to computed member expressions

[Rich-Harris]

Fixes #602. This PR allows binding to computed member expressions:

<select bind:value='prop'>
	<option>foo</option>
	<option>bar</option>
	<option>baz</option>
</select>

<input bind:value='obj[prop]'>

<pre>
{{JSON.stringify(obj)}}
</pre>

Demo (won't work until this is released...)

Probably most useful when binding to properties that can't be written using dot notation where you don't control the structure of the data (user["last-name"] or whatever), but will work with arbitrarily complex member expressions.