[TODO] openDefaultBrowser now works on OSX

[timotheecour]

• openDefaultBrowser now works on OSX
• fixes misc issues timotheecour/Nim#6 (comment)
• i checked, this was the only misuse of macos in Nim code

TODO for future PR's

openDefaultBrowser is still too naive though (and not just on OSX)

examples on OSX:

• open nimdoc.out.css opens in your editor instead of browser
• open -a 'Google Chrome' nimdoc.out.css works (but see below...)
• open ~/data/chestnut-horse-autumn_1000.jpg will not open in the browser but in your configured image editor.
• open -a 'Google Chrome' ~/data/chestnut-horse-autumn_1000.jpg doesn't even work
• open -a 'Safari' ~/data/chestnut-horse-autumn_1000.jpg works
• this seems the most reliable way to get the user's browser:

https://github.com/kerma/defaultbrowser/blob/master/src/main.m (single file, could be compile'd in in objc mode)
see also https://stackoverflow.com/questions/17528688/set-default-web-browser-via-command-line
which we could use via open -a $app $file

so at least it would fix some issues like opening files in your browser instead of editor, but not sure about the jpg example

• there should be an option to not ignore error (ignoring errors is pretty much guaranteed to cause hard to find bugs; specific applications can always try/catch)

EDIT: ok this seems to be the only thing that works reliably to open a jpg on browswer:
open -a 'Google Chrome' -n file:///Users/timothee/data/chestnut-horse-autumn_1000.jpg
any removed option (removing file://, removing -n or removing -a 'Google Chrome') will fail

but 'Google Chrome' should be replaced by auto detection