text/template: add break and continue support

[bep]

This is a repost of #20523 with, hopefully, a more clear description.

Go templates currently do not support the break nor the continue keyword -- which surprises people trying a construct similar to the one below.

{{ range $i, $e := .SomeSlice }}
  {{ if gt $i 2 }}
    {{ break }}
  {{ end }}
  {{ $e }}
{{ end }}

/cc @igramnet

[odeke-em]

/cc @rsc @robpike and other text/template folks.

[rsc]

It does seem a little reasonable. I wouldn't want labels (or goto), though. Thoughts, @robpike?

[robpike]

It does seem reasonable but low priority.

[rsc]

CLs welcome for Go 1.10. Thanks.

[gopherbot]

Change https://golang.org/cl/66410 mentions this issue: text/template: add break, continue actions in ranges

[gopherbot]

Change https://golang.org/cl/92155 mentions this issue: text/template: revert CL 66410 "add break, continue actions in ranges"