Make maximum fuzzy match length configurable

When using the fuzzy matcher, if the length of the pattern string is longer than 20 then it falls back to matcher_glob. When trying to open a file that is several directories below the current one this limit can be easily reached, which makes the fuzzy matcher less useful (to the user this also looks like a bug, because the fallback is silent).

This commit makes the maximum length configurable, and adds a note in the documentation that explains this behavior. I would also change the default to something longer, but I did not know the reason for setting it to a low value in the first place.