Add a wrap module

[TimothyGu]

Allows one to do:

var jadeSrc = 'p= body';
// By default compileClient automatically embeds the needed runtime
functions,
// rendering this module useless.
var compiledCode = jade.compileClient(jadeSrc, {
  externalRuntime: true
});
//=> 'function template (locals) { ... jade.escape() ... }'

var templateFunc = wrap(compiledCode);
templateFunc({body: 'Hey!'});
//=> '<p>Hey!</p>'

There are plenty of places where this can be used:

• https://github.com/jadejs/jade/blob/master/lib/index.js#L178
• https://github.com/jadejs/jade/blob/master/test/run.js#L70-L78
• https://github.com/jadejs/jade/blob/master/test/jade.test.js#L1069-L1103

[alubbe]

Code LGTM. I was thinking about the p= body example. I found it somewhat confusing because body is also an HTML tag. Could we change it to something else, maybe h1 Hello #{name}?