Add count option to report_lint matcher

In order to writes tests that ensure multiple lints are reported for a
single line, add a `count` option to the `report_lint` matcher so that
one can write:

    it { should report_lint count: 4, line: 2 }

...to indicate that four lints are expected to be reported for line 2.

This also changes the behaviour of `report_lint` to now check if at
least one lint was reported, rather than exactly one. This makes more
sense, since otherwise the `should_not` case would only tell you that
the linter didn't report exactly one lint (it reported zero or more than
one).

Change-Id: I1dca4349bc938332c427c35fb46954fcc4a5458d
Reviewed-on: https://gerrit.causes.com/27811
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>