Checking in changes prior to tagging of version v0.4.0

## [v0.4.0](v0.3.1...v0.4.0) (2019-04-21)

* introduce Go Modules [#21](#21) ([Songmu](https://github.com/Songmu))

Checking in changes prior to tagging of version v0.3.1

Checking in changes prior to tagging of version v0.3.0

Checking in changes prior to tagging of version v0.2.1

Checking in changes prior to tagging of version v0.2.0

Merge pull request #6 from Songmu/support-foreground

[incompatible] Support Foreground option

Merge pull request #2 from syohex/fix-document

Fix document