Misc doc changes

[kianmeng]

Besides other changes, this commit ensures the generated HTML doc for HexDocs.pm will become the main source doc for this Elixir library which leverage on ExDoc features.

List of changes:

• Use common source url
• Refactor project config
• Update license year
• Fix markdowns
• Set and use latest ex_doc
• Update matching Elixir version
• Update gitignore
• Add formatter config
• Set output format to html
• Badge and more badges!
• Revise content

Screenshot:

[kianmeng]

@danhper Any updates on this?