Merge branch 'master' of github.com:fluent/fluent-bit

Debian: bump version to v0.5.1

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

Docs: add ElasticSearch plugin documentation

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

stats: remove unused variables

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

lib: mbedtls: do not install static library files

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

Engine: fix header inclusion

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

Debian: add packager script

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>