How to handle reserved filenames in Windows?

[joeytakeda]

We now have Windows support (#250 and #33), but we will need to handle reserved filenames in Windows. As @martindholmes wrote:

List of reserved windows filenames:

CON, PRN, AUX, NUL
COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9
LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9

If files called e.g. "con.json" are created, they can't be deleted by an ordinary user. No idea what we might do here, other than add these things to the stopword list if you're building on Windows.

[martindholmes]

I think we could silently add ones with numerals to the stopword list at build time, since they're not really words, but the first four might require an explicit warning.