Add description of tokens

[kbenoit]

Also:

• breaks the lines for the paragraphs I edited, since this makes viewing git diffs easier
• formats package names using the .Rd convention of \pkg{packagename} which used boldface. Not everyone agrees with this, but the paper should at least be conistent, and currently it used either plain font or the code font. Choose one.
• Other very minor edits, e.g. capitalizing "Tweets"

[codecov-io]

Codecov Report

Merging #65 into joss-paper will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##           joss-paper      #65   +/-   ##
===========================================
  Coverage       99.43%   99.43%           
===========================================
  Files              12       12           
  Lines             351      351           
===========================================
  Hits              349      349           
  Misses              2        2

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