New UI for Prep Stage

[kirtansakariya]

Instead of having an only command line based prep stage, we're trying to making an interface that's more like a command line GUI. This will make it less confusing when going through the prep stage as well.
We'll be using asciimatics for this. More specifically, it'll be something similar to this. https://asciimatics.readthedocs.io/en/stable/widgets.html

[JaimieMurdock]

Here's a mockup of the first page

                         topicexplorer prep

                  [ 35.2% ] High frequency word filter
                  [  5.7% ] Low frequency word filter
                  [       ] Language-specific stopwords
                  [   3   ] Minimum word length

    Original corpus size:  258765 occurrences      18972 words
    Prepped corpus size:   151872 occurrences       6879 words

                       [ prep ]   [ exit ]

Here's a mockup of the frequency filters:

                           High frequency word filter

      Top % of corpus                                 # words     Rate
[ ]   20.8% ███████                                     1 words >= 12856x
[x]   35.2% ████████████                                2 words >=  2182x
[ ]   37.6% █████████████                               6 words >=  1742x
[ ]   40.0% ██████████████                             14 words >=  1352x
[ ]   42.5% ███████████████                            24 words >=   949x
[ ]   45.0% ████████████████                           39 words >=   700x
[ ]   47.5% █████████████████                          60 words >=   554x
[ ]   50.0% ██████████████████                        104 words >=   456x
[ ]   55.0% ███████████████████                       210 words >=   317x
[ ]   60.0% █████████████████████                     377 words >=   217x
[ ]   70.0% █████████████████████████                 970 words >=   106x
[ ]   75.1% ███████████████████████████              1490 words >=    71x

                    [ OK ]    [  Cancel  ]   [  Reset  ]

The frequency tables are currently defined in topicexplorer.prep.get_high_filter. I should've made a general table-view function, but as it stands, there's a lot of messy print commands.