adds full underscore style templating engine

[brianleroux]

this PR tweaks the proxy templating helper so it can execute arbitrary js and html escape values

the following index.html

<div>${ 'h'.toUpperCase() + 1 }</div>

renders

<div>H1</div>

all orig syntax remains supported

<div>${ arc.static('path/to/file.js') }</div>