Add 'requireRiot' configuration parameter.

[jrosiek]

I am not sure no one mentioned yet an issue about importing generated tags as modules. When I simply generate the tags and let brunch bundle them into single js file, the tags cannot be imported. Tag code looks like this:

riot.tag2(.......)

so a riot symbol must be in scope. It is not by default (tag is encapsulated inside of its own module). The only solution right now is to manually add a globally-scoped riot symbol. It is not a nice solution IMO, because it breaks encapsulation.

The solution which is proposed in this PR is to let the riot-brunch plugin generate necessary import inside of tag module. This is similar to -m flag in riot-cli. Unfortunately we cannot reuse this logic because -m is handled in the riot-cli, not riot-compiler.

Commit comment follows: // ---------------------

Right now when brunch wraps riot-generated code into module, that module does not have a 'riot' symbol defined. This causes runtime errors in the browser when the tag is loaded/imported.

If the 'requireRiot' parameter is set to 'true' following statement will be prepended to the compiled template:

var riot = require('riot');

This way, when the brunch wraps the tag code into isolated module, 'riot' symbol will be defined inside of that module.