Locale best match

When the browser has `en_US` as configured locale and config.yml is:
```yaml
lunetics_locale:
  strict_mode: false
  allowed_locales:
    - en
    - de
```
The matched locale will be: `en_US`. Sometimes this may cause some problems, especially if using `{_locale}` inside urls.

An alternative way can be set `strict_mode` to `true`, but in this case i will loose the possibility to match partial locales... For example, when the browser has `en_US` as configured locale and config.yml is:
```yaml
lunetics_locale:
  strict_mode: true
  allowed_locales:
    - en
```
In strict mode, no locale will be matched (fall-back to symfony default locale)...

To solve this have added a configuration called `strict_match` (default to `false`) to allow partial matching of a locale. For example:
```yaml
lunetics_locale:
  strict_mode: false
  strict_match: true
  allowed_locales:
    - en
    - en_GB
```
- When the browser has `en_US` ( or just `en`), the matched locale will be: `en`
- When the browser has `en_GB`, the matched locale will be: `en_GB`.