Lazy load themes

[Enselic]

To further improve upon #951: It is quite easy to start lazy-loading themes, and it can be done without any changes to syntect.

It makes bat start ~25% faster when syntaxes are also lazy loaded.

This might feel like overkill, but it is necessary if we want to be able to add new themes without feeling guilty about impacting startup time negatively.

Running hyperfine on bat --color=always tests/benchmarks/test-src/small-CpuInfo-file.cpuinfo:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
git master	99.6 ± 1.6	97.0	103.0	6.74 ± 0.35
git master + lazy themes	95.8 ± 1.0	94.5	98.6	6.49 ± 0.32
git master + lazy syntaxes	19.4 ± 0.9	18.0	22.0	1.31 ± 0.09
git master + lazy themes + lazy syntaxes	14.8 ± 0.7	13.5	16.5	1.00

[Enselic]

Oops, forgot CHANGELOG.md. Will fix that after the first review round.

[sharkdp]

Very cool! ❤️