[WIP] Add Porter Stemming Algorithm

[greenat92]

Hi @kariminf,
This PR for #12, Needs reviewing, All tests are passed as well.

To validate this implementation,I rely largely on these resources:

1. Porter Stemming Algorithm
2. Porter stemmer pure js Implementation

[kariminf]

Great job (y)

As for stemmer choice, It will be a little more different, like:

var EnMorpho = JsLingua.getService("Morpho", "eng");
var morpho = new EnMorpho();

morpho.setStemmer("PorterStemmer");
var stem = morpho.stem("extraordinary");

I'll do it since some changes have to be made on the parent lass

[greenat92]

@kariminf , i have problem of rebasing sorry , i'll fix it as soon as