Description
Make the AsciiSlugger's symbolsMap apply to all locales for a given language.

Example
When using the AsciiSlugger under the 'en_GB' locale, it doesn't change '&' to 'and', it removes it. However it does do this if your locale is 'en'.

Proposed approach
If the current locale is not explicitly in the symbolsMap, explode the locale string on underscores and apply the symbolsMap for the first part. E.g. for a locale of 'en_GB', it would use the 'en' symbolsMap.