set default watch directory, honor Asciidoctor options

- watch the current directory by default
- set default options for Asciidoctor
- forward options passed to guard on to Asciidoctor
- update README with new usage instructions
- rename Asciidoc to AsciiDoc where appropriate
Merge pull request #1 from mojavelinux/options-and-defaults

Set default watch directory, honor Asciidoctor options