Use go template for output

[piyush-garg]

In the pipeline describe and pipeline run describe, we are defining constants and using them later for the table header and body while printing the output.

1. cli/pkg/cmd/pipeline/describe.go

   Line 31 in 8ef40b2

   const (

2. cli/pkg/cmd/pipelinerun/describe.go

   Line 30 in 8ef40b2

   const (

we can use https://golang.org/pkg/text/template/ for templatizing the output

[vdemeester]

/kind feature