added fuzz test to try many inputs to reproduce panics

output of this test confirms some problematic inputs

$ go test -v -test.run=TestStemFuzz
=== RUN TestStemFuzz
--- FAIL: TestStemFuzz (13.02 seconds)
	porterstemmer_fuzz_test.go:24: StemString panicked for input 'eed'
	porterstemmer_fuzz_test.go:24: StemString panicked for input 'eeds'
	porterstemmer_fuzz_test.go:24: StemString panicked for input 'eing'
	porterstemmer_fuzz_test.go:24: StemString panicked for input 'eings'
FAIL
exit status 1
FAIL	github.com/mschoch/go-porterstemmer	13.048s
fix panics on certain inputs triggered in rule 5a
originally reported in identified by https://github.com/reiver/go-porterstemmer/issues/4
ran go fmt for my own sanity