make 'ignore missing' an option on Swig Options object

[steevhise]

It seems reasonable to want an "ignore missing" setting that is more general, in other words that can apply everywhere and not need to specify it for each specific use of the "include" tag. Perhaps it should even apply to entire templates that are missing. Why not make it an option to pass to the Swig Options object?

[cdaringe]

similar to #45, i somewhat think this is a preprocessing concern. im not super against it, but im having a little bit of skepticism seeing it as swig's responsibility. maybe you can paint a use case where it really helps/would-help to sell it?

[steevhise]

I admit that applying to an entire template is perhaps a stretch. But if there's already an "ignore missing" option for the include tag, it seems like having a global setting for all include tags wouldn't be much of an additional performance hit. ( One use case is having a set of template partials that I want to optionally bring in from another package. I'd want the templates that include them to not fail if that package isn't present, without having to go in and add "ignore missing" to every include tag. )

[steevhise]

I've written a custom template loader that does this, as well as #45. So no worries if y'all don't want to implement these.

[steevhise]

Here's a gist of our custom template loader that basically solves both this and #45: https://gist.github.com/steevhise/2ddac162d43c3a6112d3e983c73b4e32