api.js parses custom data attributes for client configuration, part of #29

All data-attributes beginning with `data-isso-` are stored in
`api.config` (without leading data-isso-). Isso tries to parse
the values with JSON (e.g. `-isso-foo="false"` returns false)
and falls back for a simple string value.
add an option to prevent Isso from appending the default CSS, closes #29
Merge branch 'feature/data-configuration'