Instead of requiring a flag to enable newline trimming, invert it so the
flag (now `--no-trim-newlines`) disables newline trimming. This way our
default behavior matches that of sh's `"$(cmd)"`.
Also change newline trimming to trim all newlines instead of just one,
again to match sh's behavior.
The `string collect` subcommand behaves quite similarly in practice to
`string split0 -m 0` in that it doesn't split its output, but it also
takes an optional `--trim-newline` flag to trim a single trailing
newline off of the output.
See issue #159.
I hope this is now complete.
Also, shorten enough descriptions to make `string match --<TAB>`
show a two column pager with 80 cols.
We really should have shown more retraint in the design of `string`,
not all of the flags required both a long and short option created.
