Support template.Options for setting up missing key behavior

[demsasha4yt]

Currently, there is no support to parse and execute templates with missing key options. https://github.com/nicksnyder/go-i18n/blob/main/v2/internal/template.go#L36C28-L36C28

As a result, it is not possible to return error if any variable is missing in render payload

Kindly consider supporting text/template options https://pkg.go.dev/text/template#Template.Option

"missingkey=default" or "missingkey=invalid"
The default behavior: Do nothing and continue execution.
If printed, the result of the index operation is the string
"".
"missingkey=zero"
The operation returns the zero value for the map type's element.
"missingkey=error"
Execution stops immediately with an error.

[nicksnyder]

Seems reasonable. If you want to open a PR I would review it.

[nicksnyder]

With 2.4.0 there is now a way to specify a Template.Option. Check out the release notes for more info.