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.

[JohanMabille]

We should probably capture std::cerr too as by default (at least on some platforms) it outputs to the same place as std::cout

[ianthomas23]

We do need a strategy for what to do with cerr, but I don't know what is best yet.

Currently cerr will be written to the normal terminal buffer, so it won't be seen whilst the alternative buffer is being used but it reappears when the alternative buffer is disabled. This is perhaps not a good solution, but I don't think it is too bad for a first implementation. Some other options:

1. Capture it and sent it to the same place as cout, it will appear in the output flow in the alternative buffer. We'd probably need to colour it to make is easily visible as it could be anywhere.
2. Capture it and display it at the bottom of the alternative buffer for better visibility, but then there might be quite a lot of information displayed.
3. Capture it and if anything at all is written to it we could avoid using the pager at all, just display the errors in the normal terminal flow.

[JohanMabille]

I have a preference for option 1, but I think we can discuss it in a dedicated issue and solve it in a dedicated PR so that it does not block this one, WDYT?