factor: --exponents: new option for printing in p^e format

factor: --exponents: new option for printing in p^e format

When factoring numbers that have a large 2^n factor, it can be hard to
eyeball just how many 2's there are. Add an option to print each prime
power factor in the p^e format (omitting the exponent when it is 1).

* src/factor.c: Add -h, --exponents option for printing in p^e format.
* doc/coreutils.texi (factor invocation): Document the new option.
* tests/misc/factor.pl: Add test case.
* THANKS.in: Add previous suggester
(https://lists.gnu.org/r/coreutils/2017-11/msg00015.html).

Suggested-by: Emanuel Landeholm <emanuel.landeholm@gmail.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>