Linting Service#cli_transcode

Fix hanging indent and remove ignore line.
Split off decoding from cli_output

Introduces a new private method called _cli_decode() that will handle
the decoding and logging of the returned output.
Extend _cli_exec() to support redirecting stderr

This will be needed for a future call to read stderr from CLI tools.
Introduce cli_output_error() for services

This will allow a service to check the stderr output of a tool and
ignore its return code.
Service implementation for Pico2Wave

Adds Small Footprint TTS to the services list in the add-on.
Add pico2wave to README

This documents it as another supported engine.
Rename pico2wave service

Let's call this SVOX Pico instead.
Web page for SVOX Pico

This one is short due to the simple nature of the service, but it gives
users somewhere to go for help.
Merge branch 'add/pico2wave' into develop

Brings SVOX Pico into the add-on.