Support non-terminal stdin reading for GETC and IN

[dxrcy]

Fixes #23
Don't use console::Term to read stdin, when stdin is not a terminal. Instead read to a single-character buffer using standard library functions.
Line-buffering is not a problem when stdin is not a terminal.

Improvements that could be made:

• Use shared console::Term and std::io::Stdin instances, instead of creating them for each GETC/IN call (stdin() creates a new handle the the global stdin mutex)
• Extract the common behaviour of GETC and IN to a function

[rozukke]

Cheers for the fix, will get it refactored out eventually 👍