Add support for marking up links to local files

[apankrat]

Add an option to have local paths, e.g. C:\Windows, to be marked up as file:/// links and handled the same way as http:// and https:// links are handled right now (pulse on hover, highlighted and clickable when the CapsLock is on).

[apankrat]

Done in 20210804.

Text enclosed in a pair of backticks (`) will be marked up as a file:/// link, with all forward slashes (\) converted to the backslashes (/).

For example

`c:\windows\foo bar.txt`

will appear as is and will link to

file:///c:/windows/foo bar.txt

The option is off by default and it is not accessible via the UI, not yet. To enable it, open browser console (usually accessible via F12), type the following command, hit Enter and then reload the page:

NB.poke('fileLinks', true)

To disable the option use

NB.poke('fileLinks', false)