Add option for symbol matching logic. (#1515)

The no_symbol_match makes command line completion a lot less useful. It
disables any matches for file names with symbols in them. This prevents
completing things like ":b foo/bar" to ":b foo/bar.txt" or ":b foo_" to
":b baz/foo_bar.txt". Add an option `disallow_symbol_nonprefix_matching`
that prevents a match if it contains a symbol and isn't a prefix match.
Make that option the default. Add the option to documentation and tests.
Add to the examples for command line setup disabling the option.

lua/cmp/config/default.lua

@@ -57,6 +57,7 @@ return function()
       disallow_partial_fuzzy_matching = true,
       disallow_partial_matching = false,
       disallow_prefix_unmatching = false,
+      disallow_symbol_nonprefix_matching = true,
     },
 
     sorting = {