allow watch for recursive not existing folder

[exx8]

Solves #872

This pull request allows user to watch specific not existing folders.
Was achieved via introducing a new function _mostSpecifiedExistingAncestor
We should consider making the function public, as users might be interested in this functionality.

[paulmillr]

We need tests for that feature. Also, ensure you conform to chokidar code style e.g. { locations.

[exx8]

I added tests.

[exx8]

I've made the changes that you asked.

[exx8]

Ping?

[paulmillr]

If i’m reading this correctly, we’ll have a huge performance degradation because of the new IO call for every file. Need to investigate this thoroughly and so some benchmarks.

[exx8]

If i’m reading this correctly, we’ll have a huge performance degradation because of the new IO call for every file. Need to investigate this thoroughly and so some benchmarks.

maybe we should use a flag to enable or disable this functionality?

[XhmikosR]

This needs a rebase. Also the curly brackets shouldn't be placed on new lines, there's an extra space in a test message and extra newlines.