Add sync mode

[burashka]

this.parse is sync method. This API doesn't need async. Please let me use it in sync.

[harttle]

The render() is async and returns a promise, thus parseAndRender() will return a promise. Actually, it's intended to be async to embrace the JavaScript way, see: #3 and #5 .

By the way, we are planning to make scopes to be async too, see #65.

[burashka]

Ok, sorry

[harttle]

Nothing to sorry about :) There's once a discussion over renderSync() method (#48). It could be possible to provide a sync version and throw an Error once async tags are called. It's a good point.