With the features the readline library provides it would be too naive to ignore.
We break out of the curses mode into the prompt and get back in.
This change introduces dependency on libncurses.

macOS 10.12.4 and many earlier versions have libncurses 5.4 with wide character
support, but not libncursesw.

Update README on dependencies

Add .Ed to match with .Bd

Convert warnings to errors on CI, or they'll go unnoticed.

Calculation can take very long. Most Unix-ish OSes have utilities to do this.

It was troubling me. Mimicking 'gg' would have been hacky and would need
additional processing... 'g' was free. Who cares even if you press twice? ;)

Test on Travis

This is consistent with the fact that custom LDFLAGS are already allowed, and
for once, it makes warning options easier to control.

Also use automatic variables $@ and $^ consistently.

Remove dependency on cut.