Gentoo support added

[powerman]

I've added Gentoo package (ebuild) in powerman overlay: https://github.com/powerman/powerman-overlay/tree/master/app-admin/git-secret (so it now can be installed on Gentoo by adding my overlay layman -a powerman and then installing by emerge git-secret).

At a glance looks like tool works okay, but one of the tests fails:

not ok 70 run 'git secret --version'
# (in test file /var/tmp/portage/app-admin/git-secret-0.5.0/work/git-secret-0.5.0/tests/test_main.bats, line 31)
#   `[ "$output" == "$GITSECRET_VERSION" ]' failed
#   PID TTY          TIME CMD
#     4 pts/10   00:00:00 sandbox
#     5 pts/10   00:00:00 bash
#    21 pts/10   00:00:00 bash
#    29 pts/10   00:00:00 make
#    43 pts/10   00:00:00 bash
#    47 pts/10   00:00:00 bash
#    48 pts/10   00:00:00 bash
#    55 pts/10   00:00:00 bash
#    56 pts/10   00:00:00 bash
#    57 pts/10   00:00:00 bash
#    58 pts/10   00:00:00 cat
#  6532 pts/10   00:00:00 bash
#  6755 pts/10   00:00:00 bash
#  6830 pts/10   00:00:00 ps

The command mentioned by test works fine too:

$ git secret --version
0.5.0