Implement terminal_pager for log subcommand

[ianthomas23]

The start of a terminal pager for various git2cpp commands, initially enabled for log subcommand. Fixes #45.

pager.mp4

Supports the following keys to navigate:

d, space                   scroll down a page
u                          scroll up a page
q                          quit pager
down arrow, enter, return  scroll down a line
up arrow                   scroll up a line

If cout is not to a tty or the output is short enough to fit within a single page the pager is not used.

Still to do, probably in separate PRs:

• Gracefully deal with lines that are longer than the terminal width.
• Searching by regex.

To add to other subcommands, see how it is used in log_subcommand.cpp. The terminal_pager constructor does the required initialisation, and it displays on the show call. I suppose I could have done the show automatically in the destructor but that didn't seem a good idea.

Awaiting a new release of JupyterLite before we can easily try it in the terminal.