Make Parsers and Compilers configurable

[ebottacin]

My Goal is to extend ngx-translate-extract to use as a library using my custom implementantion of Parsers and compilers that extends the standard implementations.

I've added a method to ParserInterface (preprocessKeys) in order to rewrite extracted keys before adding them to TranslateCollection. In the standard Parser the method implementation simply return the input.

In order to load my custom implementations i've used InversifyJS: the default implementations is still the same but i can write my custom app with ngx-translate-extract as dependency and plug my custom implementations.

this is done by adding to cli.js some configuration lines

`#! /usr/bin/env node

//need to be imported once per application: @see inversify/InversifyJS#262 (comment)
let reflectMetadata = require('reflect-metadata');

//IoC configuration: imported here to allow redefintion for usage as library
let inversifyConfig = require('../dist/ioc/inversify.config');

let inversifyConfigParsers = require('../dist/ioc/inversify.config.parsers.custom-impl');

let container = inversifyConfig.default;
inversifyConfigParsers.configParsers(container); // load custom parsers
inversifyConfigParsers.configCompilers(container); // load custom compilers

let cli = require('../dist/cli/cli');

cli.extractTask.execute();
`

Do you think it's possible to merge this?