[Feature] Add a --query option to the CLI

Hello,

Before anything, I'd like to say thank you to everyone who contributed, since this piece of software has immensely improved my XPS 15 battery life without the usual complexity, enabling me to work for much longer hours without charging.

I'm opening this feature request to propose a `--query` option on the CLI to enable users to obtain a specific value in plain text format. The use-cases I have for this is scripting and UI programming (custom widgets, for example). Autocomplete could be leveraged to provide the fields which can be queried.

This could look like :
Command : `auto-cpufreq --query CPUAvg`
Output : `42.67`

If somebody has the time to quickly implement this (I assume it should be pretty straightforward to add), please go ahead; otherwise, I may do it myself in a couple of weeks/months, when I have more time ahead of me to dive into this project.