v1.13.3-alpha: Make stern compatiable with OAM vela

Replace the`github.com/wercker/stern` with
`github.com/zzxwill/stern` in go.mod, and remove short hand flag `e` as it
will conflict with OAM vela

v1.13.2: Upgrade depended client-go version

upgraded depended client-go version from
v0.0.0-20190620085101-78d2af792bab to v0.18.6

v1.13.1: Split command generation and execuation in cli.go

split cli.Run() to have it returned *cobra.Command and call
Execute() in main.go. This can make it integrable by outside
tool