Add option for file permission

[sushichop]

This PR refers #35.

I have added an option for file permission.
You can use this feature as follows.
Default permission is "640"

// e.g. 1
FileLogger("com.example.yourapp.file",
                   fileURL: "./yourlog.log",
                   filePermission: "600")
// e.g. 2
try! FileRotationLogger("com.example.yourapp.filerotation",
                                        fileURL: "./logs/foo.log",
                                        filePermission: "660")

[codecov]

Codecov Report

Merging #36 (1a44416) into main (0d5f9f1) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   95.83%   95.93%   +0.09%     
==========================================
  Files          11       11              
  Lines         408      418      +10     
==========================================
+ Hits          391      401      +10     
  Misses         17       17              

Impacted Files	Coverage Δ
Sources/Puppy/FileError.swift	100.00% <ø> (ø)
Sources/Puppy/FileLogger.swift	93.00% <100.00%> (+0.77%)	⬆️
Sources/Puppy/FileRotationLogger.swift	86.11% <100.00%> (ø)

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