feature: Add persistent search settings

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

Adds persistent search settings across runs, by saving to the config file. Each process widget keeps track of it's own behaviour. The previous flags/options are now for global behaviour.

The following new behaviour is:

• Relevant flags: --case_sensitive, --whole_word, and --regex, will override the current widget's default behaviour.
• Relevant options: case_sensitive, whole_word, and regex, will also override the current widget's default behaviour.

As per before, if you set, say, --case_sensitiveand case_sensitive=true, the flag always overrides.

Issue

If applicable, what issue does this address?

Closes: #243

Type of change

Remove the irrelevant ones:

• New feature (non-breaking change which adds functionality)
• Breaking change (if this change causes breakage or new behaviour, please state what)

Test methodology

If relevant, please state how this was tested:

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

If relevant, all of these platforms should be tested.

Checklist

If relevant, ensure the following have been met:

• Change has been tested to work, and does not cause new breakage unless intended
• Code has been self-reviewed
• Documentation has been added/updated if needed (README, help menu, etc.)
  To do in Add documentation for in-app config #248.
• Passes Travis tests (clippy check and cargo test check)
• Areas your change affects have been linted using rustfmt (cargo fmt)
• No merge conflicts arise from the change

Other information

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