Make it possible to write to stdout instead of to file-system

[MatthiasMargot]

Unfortunately, I can't make an argument conditionally optional in commander.

I wanted to do this:

• the 2nd argument, pathToDestination, is required when the flag -l, --log is not provided
• if -l, --log is provided only pathToSource is required and the output is simply logged to the console

Commander doesn't allow this, an argument is either optional or required, not conditionally so. I could throw a custom error but in that case, the auto-generated help message that says that pathToDestination is optional would not be correct, it wouldn't reflect the fact that it's optional only sometimes.

Therefore I went for an inversion of this API:

• adding an option -o, --output which takes the pathToDestination
  • making only pathToSource required
• making the command log back to stdout by default

Unfortunately, this inverses the default behavior from file-system write to stdout write but we get the desired functionality and the help & documentation are consistent with the behavior.

Fixes #19

[codecov[bot]]

Codecov Report

Merging #20 (d4f919d) into master (45db1c4) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #20   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines           29        29           
  Branches         3         3           
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  Hits            29        29           

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