Support join and sort kwargs to both readdir and readpath

[rofinn]

Closes #110

julia> using FilePathsBase

julia> readdir(cwd())
12-element Vector{String}:
 ".DS_Store"
 ".git"
 ".github"
 ".gitignore"
 "LICENSE.md"
 "Manifest.toml"
 "Project.toml"
 "README.md"
 "docs"
 "foo.txt"
 "src"
 "test"

julia> readdir(cwd(); join=true)
12-element Vector{String}:
 "/Users/rory/repos/FilePathsBase.jl/.DS_Store"
 "/Users/rory/repos/FilePathsBase.jl/.git"
 "/Users/rory/repos/FilePathsBase.jl/.github"
 "/Users/rory/repos/FilePathsBase.jl/.gitignore"
 "/Users/rory/repos/FilePathsBase.jl/LICENSE.md"
 "/Users/rory/repos/FilePathsBase.jl/Manifest.toml"
 "/Users/rory/repos/FilePathsBase.jl/Project.toml"
 "/Users/rory/repos/FilePathsBase.jl/README.md"
 "/Users/rory/repos/FilePathsBase.jl/docs"
 "/Users/rory/repos/FilePathsBase.jl/foo.txt"
 "/Users/rory/repos/FilePathsBase.jl/src"
 "/Users/rory/repos/FilePathsBase.jl/test"

julia> readpath(cwd())
12-element Vector{PosixPath}:
 p"/Users/rory/repos/FilePathsBase.jl/.DS_Store"
 p"/Users/rory/repos/FilePathsBase.jl/.git"
 p"/Users/rory/repos/FilePathsBase.jl/.github"
 p"/Users/rory/repos/FilePathsBase.jl/.gitignore"
 p"/Users/rory/repos/FilePathsBase.jl/LICENSE.md"
 p"/Users/rory/repos/FilePathsBase.jl/Manifest.toml"
 p"/Users/rory/repos/FilePathsBase.jl/Project.toml"
 p"/Users/rory/repos/FilePathsBase.jl/README.md"
 p"/Users/rory/repos/FilePathsBase.jl/docs"
 p"/Users/rory/repos/FilePathsBase.jl/foo.txt"
 p"/Users/rory/repos/FilePathsBase.jl/src"
 p"/Users/rory/repos/FilePathsBase.jl/test"

julia> readpath(cwd(); join=false)
12-element Vector{PosixPath}:
 p".DS_Store"
 p".git"
 p".github"
 p".gitignore"
 p"LICENSE.md"
 p"Manifest.toml"
 p"Project.toml"
 p"README.md"
 p"docs"
 p"foo.txt"
 p"src"
 p"test"

The defaults maintain the current no-kwarg behaviour and type stability. We may unify readpath and readdir in the future now that readdir can do what we wanted readpath for. However, that would be a breaking release.

[codecov]

Codecov Report

Merging #152 (0639b4b) into master (6f003d8) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
+ Coverage   91.88%   91.91%   +0.02%     
==========================================
  Files          12       12              
  Lines        1208     1212       +4     
==========================================
+ Hits         1110     1114       +4     
  Misses         98       98              

Impacted Files	Coverage Δ
src/path.jl	87.90% <100.00%> (+0.16%)	⬆️
src/system.jl	92.46% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6f003d8...0639b4b. Read the comment docs.