Support multiple configs with .erdtree.toml

[solidiquis]

Closes #159

What's new

This PR introduced the following key-value pair to the list of command-line arguments:

-c, --config <CONFIG>  Use configuration of named table rather than the top-level table in .erdtree.toml

In addition to supporting a .erdtreerc configuration file, erdtree now supports a .erdtree.toml TOML file as well. The .erdtree.toml file, unlike .erdtreerc, supports multiple configurations that you can specify using named tables. Here is an example of a valid .erdtree.toml file:

icons = true
human = true

# Compute file sizes like `du`
[du]
disk-usage = "block"
icons = true
layout = "flat"
no-ignore = true
no-git = true
hidden = true
level = 1

# Do as `ls -l`
[ls]
icons = true
human = true
level = 1
suppress-size = true
long = true

The top-level table only contains two arguments: icons = true and human = true. Without providing any arguments to -c, --config, the top-level table will get invoked which, after processing, will get transformed into erd --icons --human. To invoke configurations from a named table instead, pass the name of the table to -c, --config. Here are some examples given the above .erdtree.toml

$ erd

# equivalant to 

$ erd --human --icons

...

$ erd -c du

# equivalent to

$ erd --disk-usage block --icons --layout flat --no-ignore --no-git --hidden --level 1

User provided arguments take precedent over config

$ erd -c du --level 10

# equivalent to

$ erd --disk-usage block --icons --layout flat --no-ignore --no-git --hidden --level 10

Config rules

• Unlike .erdtreerc, .erdtree.toml only accepts long-named arguments without the preceding --. Types are enforced, so numbers are expected to be numbers, booleans are expected to be booleans, strings are expected to be strings, and so on and so forth.

• If you have both a .erdtreerc and a .erdtree.toml, the .erdtreerc will be prioritized and .erdtree.toml will be ignored. There is really no good reason for you to have both. Supporting .erdtreerc hence-forth will be for backwards compatibility.

• This is the order wherein erdtree looks for the .erdtree.toml file on Unix platforms:

/// - `$ERDTREE_TOML_PATH`
/// - `$XDG_CONFIG_HOME/erdtree/.erdtree.toml`
/// - `$XDG_CONFIG_HOME/.erdtree.toml`
/// - `$HOME/.config/erdtree/.erdtree.toml`
/// - `$HOME/.erdtree.toml`

and on Windows:

/// `%APPDATA%/erdtree/.erdtree.toml`