Add exemption for type assertions in Is(error) bool methods

[triarius]

Fixes #50

[triarius]

PS I tried to run the tests locally and was met with some resistance:

?   	github.com/polyfloyd/go-errorlint	[no test files]
src/allowed/allowed.go:15:2: cannot find package "golang.org/x/sys/unix" in any of:
	/usr/lib/go/src/golang.org/x/sys/unix (from $GOROOT)
	/home/narthana/devel/triarius/go-errorlint/errorlint/testdata/src/golang.org/x/sys/unix (from $GOPATH)
/home/narthana/devel/triarius/go-errorlint/errorlint/testdata/src/allowed/allowed.go:15:2: could not import golang.org/x/sys/unix (invalid package name: "")
--- FAIL: TestAllowedComparisons (0.45s)
    analysistest.go:304: error analyzing errorlint@allowed: analysis skipped due to errors in package
FAIL
FAIL	github.com/polyfloyd/go-errorlint/errorlint	1.617s
FAIL

[kolyshkin]

PS I tried to run the tests locally and was met with some resistance:

@triarius mea culpa. Broken by #47, fixed in #52. Missed because CI was not running tests (fixed in #53). Quite a story :)

[kolyshkin]

@triarius you also need to write commit message titles in the commitizen format.

[triarius]

No worries @kolyshkin

I have cherry-picked your commit and the tests run fine locally.

[polyfloyd]

Thanks!