2.0.0-beta.3

V2 (#10)

* LICENSE: change to MIT

* ci: only build Go 1.11 or above

* v2: basic logic and tests

* v2: add console mode

* v2: remove custom errors

* v2: add slack mode

* v2: add discord mode

* v2: add file mode

* v2: enable Go Modules

* v2: make channel loop as a manager method

* v2: add test for file mode

* README: update for v2

* Update README.md

* Update .travis.yml

* Update .travis.yml

* Update README.md

* v2: add more tests to slack and discord loggers

* Update codecov.yml

* Rename codecov.yml to .codecov.yml

* clog: use chanLogger to replace bufLogger

* clog_test: remove debug lines

* Fix tests

* logger: print error message when no logger is available

* v2: improve comments and golint

* Update README.md

* v2: fix module path