Create config-level entry to enable filtering items.

[el-tocino]

Within the mycroft conf you should be able to add local config entry to enable/disable denoise/highpass/lowpass/normalize/other? filters. Right now they're all just on by default in local deepspeech server config. They could be used on other engines as well, which I have not tested with those adjustments, if one was so motivated to try*. Performance considerations might also change what people want to have filtered. The high/low pass frequencies would also be nice things to allow config settings for.

• this is left as an exercise for the reader.