Add logic to update mismatching PR dictionary for opened and rebased PRs

Once dictionaries have been checked and missing comments have been identified,
the dictionaries run through an update_dictionary() method which goes through
each of the unfound keys in the primary PR dictionary, checks wether the PR is
open and parses the body/comments for a rebase note
Also add a parse() method to PullRequest that parses the PR body first then
parses the comments if nothing is found.