Allow a custom location for the filters file

You can now add a key 'filters' in the preferences file that points to the
path of the filters file.  If this key is not found in the preferences,
the default location '.duplicacy/filters' is used.

There is a new option '-filters' for the set command that set this key in
the preferences, but you can also edit the file directly.

src/duplicacy_preference.go

@@ -25,6 +25,7 @@ type Preference struct {
 	DoNotSavePassword bool              `json:"no_save_password"`
 	NobackupFile      string            `json:"nobackup_file"`
 	Keys              map[string]string `json:"keys"`
+	FiltersFile           string        `json:"filters"`
 }
 
 var preferencePath string