I though plain `read` had usurped them, but when searching a very small
number of files, mmaps can be around 20% faster on Linux. It'd be really
unfortunate to leave that on the table.
Mmap searching doesn't support contexts yet, but we probably don't really
care. And duplicating that logic doesn't sound fun. Without contexts, mmap
searching is delightfully simple.
- Refactored interaction between CLI args and rest of xrep.
- Filling in a lot more options, including file type filtering.
- Fixing some bugs in globbing/ignoring.
- More documentation.
