Add `BufRead` extension methods for reading text terminated by a `NewlinePattern`

[jwodder]

Prerequisite: #10

• There should be NewlinePattern equivalents of both BufRead::read_line() and BufRead::lines() (and BufRead::read_until() and BufRead::split()?)

• Add options for whether to retain trailing matches and/or yield a final empty string after a final match