feat: Optionally write to a buffer instead of stdout

[Piturnah]

Following on from my comment in #956, this PR adds the ability to write output to a std::fmt buffer instead of stdout. It does this by changing the handle used by the printer to be a mutable reference to either one of dyn fmt::Write or dyn io::Write.

Included is also an added example to show that it works. I didn't add it to the PrettyPrinter but I can do if it seems like a good idea.

Anything else let me know!

[sharkdp]

Can we please run a simple benchmark (e.g. bat large-plaintext-file.txt) to make sure this doesn't lead to any regression.

[Piturnah]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
bat-main	797.0 ± 6.1	790.5	817.9	1.01 ± 0.01
bat-2618	792.9 ± 2.6	788.7	800.6	1.00

Both compiled in release mode, bat-main comes from 8676bbf (the commit this PR is based on)

5 warmup runs, 20 runs

Command used to generate the file:
tr -dc "A-Za-z 0-9" < /dev/urandom | fold -w100|head -n 1000000 > large-plaintext-file.txt

[sharkdp]

Thank you very much!

tr -dc "A-Za-z 0-9" < /dev/urandom | fold -w100|head -n 1000000 > large-plaintext-file.txt

❤️

[sharkdp]

I'm just rebasing to current master to see if the tests still pass.