resolves #218 allow round bracket in link

- switch to strategy of fixing up round brackets in subsitution method
resolves #213 support email links and mailto macros
Merge pull request #219 from mojavelinux/bracket-in-link

resolves #218 allow round bracket in link
Merge pull request #234 from mojavelinux/mailto

resolves #213 support email links and mailto macros
recognize opts as shorthand for options

- allow opts attribute to be used as a shorthand to options
- set the default value of each option to empty string instead of nil
- document this feature in the differences from AsciiDoc section of the README