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.

doc/cmp.txt

@@ -123,7 +123,8 @@ A recommended configuration can be found below.
         { name = 'path' }
       }, {
         { name = 'cmdline' }
-      })
+      }),
+      matching = { disallow_symbol_nonprefix_matching = false }
     })
 
     -- Setup lspconfig.
@@ -560,6 +561,11 @@ matching.disallow_prefix_unmatching~
   `boolean`
   Whether to allow prefix unmatching.
 
+                                cmp-config.matching.disallow_symbol_nonprefix_matching
+matching.disallow_symbol_nonprefix_matching
+  `boolean`
+  Whether to allow symbols in matches if the match is not a prefix match.
+
                                             *cmp-config.sorting.priority_weight*
 sorting.priority_weight~
   `number`