Change source of emojis to github api so GFM can support all possible github emoji shortcodes (dart-lang/markdown#427)

* change source of emojis to github api

* revise command line options

* dart format

* remove srawlins todo as it is no longer applicable, replace with comment about github

* update with run of update_github_emojis.dart

* change return to exit(0) to exit

* standardized use of GitHub, API and Unicode case, and emoji as plural, except for when refering to emojis.dart file

* standardized use of GitHub, API and Unicode case, and emoji as plural, except for when refering to emojis.dart file

* dart formatting trailing space

* fix comments to be dartdoc

* cleanup dartdoc comments

* furthe comment clarification, fix variable names

* clarified comment

* finished missed changes

* dart format

* tooltip wording fix

* variable declaration simplifications

* clarify comment

* dart formatting