Support cspell like custom local dictionaries

[mayromr]

This is my hack to implement this feature given the current dictionary management, explained in #85. I don't think it's polished yet (hence the draft pull request), I'll add a to-do list with issues that I think I need to solve before this can get merged.

The TLDR is that now you can add custom dictionaries definitions in the codebook.toml like so:

dictionaries  = ["relative"]

[[custom_dictionaries_definitions]]
name = "relative"
path = "./relative.txt"
allow_add_words = true

Also this feature lets the user add local dictionaries in a round about way.

TO-DO:

• Update docs and readme.
• move file access from codebook/src/lib.rs to a helper.
• rethink try_normalizing_relative_paths, could cause bad times if we ever write a relative path back as absolute.
• replace raw file access with WatchedFile when updating custom dictionaries.
• Filter out custom dictionaries with non standard extension ( might not be the best idea ) .
• Test on local machine.

I feel like the main issue is state management, I want to change DictionaryManager to hold concrete implementations of Dictionary or maybe add a new add a new editable_dictionaries field that would hold all the editable dictionaries and there respective WatchedFile. I would love to hear what you have to say.

[ccoVeille]

cc @Jason3S cspell maintainer