Quote fewer characters, i.e. :, @, and + - #44

Previously any one of these dragged a whole word into quotes, so a URL came out
as `https'://github.com/allenap/shell-quote'`. That is correct but noisy, which
rather undermines the claim that `Sh` output is "better for humans to read, to
copy and paste".

All three are inert wherever they land in a word, checked in leading, medial,
and trailing position against `/bin/sh`, Bash 3.2 and 5.3, Dash, Z Shell 5.9,
and fish 4.8.

`%` and `=` were tried too, and would have addressed #8 as well, but they
earned a guard that grew a new special case each time I looked at it – append
assignments, `=` expansion, `MAGIC_EQUAL_SUBST`, job specifications – and two
of those were found only after I had convinced myself the thing was correct.
For a crate whose one job is to emit a single literal word, terser output is
not worth that. They stay quoted, and the module documentation in `ascii.rs`
now records every hazard found, and the reason a plain non-interactive `sh -c`
probe fails to reveal them, so that the next person to try this starts where I
left off rather than where I began.

Closes #42.

In command position Bash reads a word beginning with `%` as a job
specification, so `%1` on its own means `fg %1`, and quoting does not prevent
it: Bash tests the word's value, after expansion and quote removal, so `%1`,
`'%1'`, `"%1"`, `$'%1'`, `\%1`, and `""%1` all alike run `fg`.

Nor does it depend on job control being enabled, which was my first guess. Bash
rewrites the word either way and only the complaint differs, `fg: no job
control` or `fg: %1: no such job`; the same holds with `set -m`, with `set +m`,
under `--posix`, and in Bash 3.2. An executable named `%1` sitting on `PATH`
will not be run, though it runs happily by an explicit path. Only the first
character counts: `a%1` is an ordinary command name.

Z Shell tests the literal token instead, so quoting does work there. That is
why `%` is quoted unconditionally rather than only when it leads a word: it is
what Z Shell needs, and it is at worst harmless in Bash.

Worth keeping in proportion: this bites only in command position. As an
argument, which is what this crate is mostly used for, `%` is an ordinary
character in both shells.