Allow for extension of lexicon

[passbyval]

Context

I was working with the tagger API, and I realized that many "tech" words aren't tagged properly. As a work-around, I'm currently doing this, but thought it would be useful to have a way to do so within the API:

import extendedLexicon from './lexicon.json'

const lexicon = new Lexicon(language, TAGS.DEFAULT)

lexicon.lexicon = {
  ...lexicon.lexicon,
  ...extendedLexicon,
}

Summary

Add 4th parameter to allow for the extension of lexicons to prevent breaking previous versions.

[coveralls]

Pull Request Test Coverage Report for Build 6318130822

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 87.349%

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Totals
Change from base Build 6148761892:	0.008%
Covered Lines:	9664
Relevant Lines:	10693

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💛 - Coveralls

[Hugo-ter-Doest]

Thank you so much for your idea and contribution. Will merge this now.