Add lookup.exec processor

[urso]

experimental lookup processor.

$ go test -cover ./...
?       github.com/elastic/beats/libbeat/processors/actions/lookup  [no test files]
ok      github.com/elastic/beats/libbeat/processors/actions/lookup/exec 0.179s  coverage: 76.1% of statements
ok      github.com/elastic/beats/libbeat/processors/actions/lookup/lutool   0.328s  coverage: 86.8% of statements

[tsg]

@urso LGTM, but there's a windows compilation error (see appveyor & travis).

[tsg]

Oops, looks like the compile error is fixed and I was looking at an old run. Waiting for green.

[tsg]

The appveyor is probably a flaky test (fixed in #2159). Merging as is.

[urso]

yeah, apparently syscall.SysProcAttr differs between different platforms.