feat: allow code snippet execution in any language

[mfontanini]

This adds the groundwork to be able to allow code execution for any supported language rather than just bash scripts by using the +exec switch. The idea is that we files in the repo's ./executors directory where the file name should be <CodeLanguage>.sh and should contain a shell script that indicates how that programming language is executed. The script will be invoked when a code snippet for that language is executed, taking as an argument the path to a file that contains the code snippet.

For interpreted languages like python the executor may be a script that contains a single line invoking the interpreter. For compiled languages it may contain an invocation to the compiler + one to the output binary.

Besides built in executors, you can define executors locally in $config_path/executors (~/.config/presenterm in linux) where each file should follow that naming format ^.

There's still more work to be done to:

1. Add a lot more executors. This simply adds python as a showcase but adding more should now be trivial for contributors.
2. Allow defining executors within the presentation's path so that you can have a self contained reproducible presentation.
3. Allow processing output in real time as you now only see output when python finishes. I think something is using buffered output and is causing us to only receive the script's output once it finishes.

Relates to #242