Multi-pass support; add missing-key option to skip over missing keys without altering the template

[loucadufault]

The use case would be to enable workflows which use gomplate to render a template over several passes, perhaps with different datasources at each pass, such that not all keys are available upon the first pass.

E.g.

$ echo 'name: Bob' | gomplate --missing-key skip -i 'Hi {{ .name }} (password: {{ env.Getenv "PASSWORD" }})' -c stdin: -o intermediate.txt
$ PASSWORD=hello123 gomplate -f intermediate.txt

The first command would generate an intermediate template, with the template for the missed key "PASSWORD" unaltered, which would allow a second pass to render it in.

Is this kind of behaviour feasible?