Additional sentiment classifiers

[dylanbouchard]

Is your feature request related to a problem? Please describe.
Currently, the SentimentBias and CounterfactualMetrics classes offer computation of counterfactual sentiment bias using only a single sentiment classifier, vaderSentiment.

For more robustness, it may be worthwhile to include additional sentiment classifiers for the user to choose among. Ideally, we could do so in a way that does not introduce new dependencies.

Describe the solution you'd like
There are multiple options for how this could be structured:

• Similar to ToxicityMetrics class that offers multiple toxicity classifiers. If users specify multiple, an ensemble is used.
• Only a single sentiment classifier may be specified
• If multiple classifiers are specified, return a value of counterfactual sentiment bias for each classifier

Describe alternatives you've considered
Status quo

[ManaliJoshi92]

I have just started working on this issue and have opened a draft PR #129
This is still a work in progress, and I will update it as I make progress.