grep: make '-o' override context arguments

[Froloket64]

Now passing -o or --only-matching ignores any context arguments, like GNU grep does.

Fixes #67 (funny).

Verification

I used this script to test correctness:

#!/usr/bin/env bash

TARGET=./target/debug/grep

printf "uutils:\n"
printf 'aa\nbb\ncc\nxx\n' | "$TARGET" -e 'aa' -o -A 2
printf 'aa\nbb\ncc\nxx\n' | "$TARGET" -e 'xx' -o -B 2
printf 'aa\nbb\ncc\nxx\n' | "$TARGET" -e 'cc' -o -C 2
printf 'aa\nbb\ncc\nxx\n' | "$TARGET" -e 'xx' -o -B 2 -n

printf "gnu:\n"
printf 'aa\nbb\ncc\nxx\n' | grep --color=always -e 'aa' -o -A 2
printf 'aa\nbb\ncc\nxx\n' | grep --color=always -e 'xx' -o -B 2
printf 'aa\nbb\ncc\nxx\n' | grep --color=always -e 'cc' -o -C 2
printf 'aa\nbb\ncc\nxx\n' | grep --color=always -e 'xx' -o -B 2 -n

[sylvestre]

please add tests in tests/

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing Froloket64:main (33ae27a) with main (670669e)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Froloket64]

Sorry, had trouble with forked branches so had to do an extra merge.