Add extensibility features to cohtml

[iShafayet]

One of the core ideas behind cohtml was to make it extensible in such a manner that it's code can be reused in other projects that seek to extend cohtml. (For example, my project 'copolymer' which adds in support for native conditional like if for etc in cohtml and transpiles to a polymer web-component)

We need to modify the api in such a way that it can be easily extended and reused.

The code needs to be separated into more chunks.