Allow customisation of the spec filename

This allows projects to follow their own file naming conventions for JavaScript specs.

For example _my-module-spec.js_ or _module.spec.js_. This is done through setting a new spec_matcher config property to a regular expression which is then used by `spec_paths` to match the files.

Usage:

```rb
Konacha.configure do |config|
  config.spec_matcher = /-spec\./
end if defined?(Konacha)
```

Default behaviour remains the same.