add Debug log call (if DEBUG=true)
add more flexibility and tests

* allow configuring where to write assets.go
* do not overwrite assets.go when exists (this should be done manually)
* scan sub-directories
* only skip the PackagePath itself
* keep prefixes in keys
* add unit tests
* add integration tests
add method to extract key=value tags from syslog lines
use Doc for indexing

* allow to index different types with one batch request