Fix to support other languages

[Relfos]

The current version has major problems for supporting non-english users.
First, it uses the current system culture while trying to parse decimals with dot, which causes an exception to be thrown when running Vader in a OS with Portuguese language (probably will fail in French and other languages).

Second, it uses an hardcoded lexicon file which is great for English but this library should also be able to analyze texts from other languages. For that, I added an second constructor that takes a file name as argument, to let the programmer load a custom lexicon file.

Finally, due that previous change, I removed the static constructor and static lexicon dictionary.
The programmer can keep an instance of an SentimentIntensityAnalyzer and reuse it as necessary. And now with the addition of custom lexicon files, we might need to have multiple lexicon files loaded, so the dictionary can no longer be static.

[codingupastorm]

Sweet thanks for this, will check it out and merge asap :)