Support template formatting for plain output

[philipsd6]

It would be nice to have the ability to format the output of jira issue list using Go templates. This is what I have to do now to get the output I need for importing in another tool:

jira issue list -q 'filter="My Filter"' \
    --plain --no-headers --columns KEYS,SUMMARY,PRIORITY \
    | awk -F'\t' '{if ($3 == "") print $1, $2; else print $1, $2, "["$3"]"}' \
    | xclip -sel clip

I would be happy to use this instead of Awk:

jira issue list -q 'filter="My Filter"' \
    --plain --format "{{.Key}} {{.Summary}}{{ if .Priority }} [{{ .Priority }}]{{else}}{{end}}" \
    | xclip -sel clip

I would expect that --no-headers would be implied by the use of --plain and --format.

Ideally, we would be able to also include desired custom fields, perhaps as {{ .Custom.story-points }} for example.