fixes #98

BREAKING CHANGE: With this release any custom configuration files will no longer
automatically extend `html-validate:recommended`.

The previous behaviour was to to always merge with the default configuration
containing `extends: ["html-validate:recommended"]`. This is counter-intuitive
when adding a blank `{}` `.htmlvalidate.json` configuration file (or a file with
`extends: []`). The new behaviour is to not apply default configuration if
another configuration is found.

To retain the previous behaviour you must ensure your configuration contains
`extends: ["html-validate:recommended"]`. Users who have `root: true` are not
affected. *If unsure: test your configuration by deliberatly introducing and
error and verify it is detected*.

For CLI users: ensure your `.htmlvalidate.json` configuration files are updated.

For IDE integration users: ensure your `.htmlvalidate.json` configuration files
are updated.

For `CLI` API users: same as with CLI the configuration loading has changed and
all users must update their `.htmlvalidate.json` accordingly.

For `HtmlValidate` API users: you are most likely not affected, only if both of
the following conditions are true you need to take care to ensure any
`.htmlvalidate.json` is updated:

1. you are using `validateFile` or supply filenames to other validation
   functions.
2. you allow user-supplied configurations (or use them yourself) via
   `.htmlvalidate.json` (default unless `root: true` is set in the configuration
   object passed to `new HtmlValidate(..)`)

The `ConfigLoader` API has also been updated and `fromTarget` may now return
`null` if no configuration file was found.