config.json: add ignore pattern
config.json: add ignore pattern

see #621
see also https://github.com/exercism/trackler/pull/33

This adds a repo specific ignore pattern. The default pattern
`/example/` excludes example test files, such as
`example_helloworld_test.go`, which should not be ignored.

This pattern ignores a filename that contains the word `example`, unless
the word `example` is followed by the word `test`, with any number of
characters inbetween.

For instance, these files will be ignored:

`example.go`
`example_slice.go` (from the `custom-set` exercise)

however these files will not be ignored:

`example_test.go`
`example_clock_test.go`
`example_helloworld_test.go`

leenipper/xgo,exercism/xgo,leenipper/xgo,robphoenix/exercism-go,robphoenix/exercism-go,exercism/xgo