support option type

[Roger-luo]

This implements the following dot syntax for long options when the corresponding type
is an option type defined by Configurations, e.g

using Test
using Comonicon
using Configurations

@option struct OptionA
    a::Int
    b::Int
end

@option struct OptionB
    option::OptionA
    c::Int
end

"""
# Options

- `-c, --config <path/to/option/or/specific field>`: config.
"""
@main function run(;config::OptionB)
    @test config == OptionB(OptionA(1, 1), 1)
end

can be used with the following syntax

command --config.c=1 --config.option.a=1 --config.option.b=1
command --config=config.toml
command -c config.toml

[codecov]

Codecov Report

Base: 83.24% // Head: 83.65% // Increases project coverage by +0.41% 🎉

Coverage data is based on head (9d2ea81) compared to base (ba45d81).
Patch coverage: 88.88% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   83.24%   83.65%   +0.41%     
==========================================
  Files          21       21              
  Lines        1540     1585      +45     
==========================================
+ Hits         1282     1326      +44     
- Misses        258      259       +1     

Impacted Files	Coverage Δ
src/codegen/julia.jl	88.85% <88.88%> (+1.49%)	⬆️

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