Support a configurable missing key action

[brikis98]

This PR adds a new flag to boilerplate called missing-key-action. This flag determines what boilerplate will do if a template references a variable that is not defined:

• error (default): exit with an error. Making this the default helps to catch a lot of bugs and typos in your templates.
• invalid: render the text <no value>.
• zero: render the zero value of the variable. For now, this will always be an empty string, but if we support types for variables, it will be different for each type.

[brikis98]

@josh-padnick This is ready for review

[josh-padnick]

I haven't reviewed the code yet, but at first blush, invalid seems like the wrong word to return a placeholder. "Invalid" makes me think that if something isn't valid, an error should be thrown. Otherwise, we're just marking something explicitly as empty. So I vote to use the word placeholder instead.

[josh-padnick]

Left minor comments but overall LGTM! Feel free to respond as desired and merge.

[brikis98]

I haven't reviewed the code yet, but at first blush, invalid seems like the wrong word to return a placeholder. "Invalid" makes me think that if something isn't valid, an error should be thrown. Otherwise, we're just marking something explicitly as empty. So I vote to use the word placeholder instead.

It's based on the names of the settings supported by Go's templating system:

"missingkey=default" or "missingkey=invalid"
    The default behavior: Do nothing and continue execution.
    If printed, the result of the index operation is the string
    "<no value>".
"missingkey=zero"
    The operation returns the zero value for the map type's element.
"missingkey=error"
    Execution stops immediately with an error.

[brikis98]

Merging now!