add MongoDB plugin

README.md

etc/config.sample.toml

plugins/all/all.go

plugins/mongodb/mongodb.go

plugins/mongodb/mongodb_data.go

plugins/mongodb/mongodb_data_test.go

plugins/mongodb/mongodb_server.go

plugins/mongodb/mongodb_server_test.go

plugins/mongodb/mongodb_test.go

plugins/mongodb/mongostat.go

testutil/accumulator.go