Collection task should work even if no :content files are found

[podviaznikov]

E.x. https://github.com/hashobject/perun.io/blob/master/build.boot#L32 this should work without creation of fake index.md

[martinklepsch]

I'm thinking the same should be true for the render task. If the user only wants to render files that contain a :content key they may use the filterer option. Hardcoded filtering like this makes it harder to use the render task to render files other than markdown. (i.e. edn)

[podviaznikov]

Here is the problem when there are not metadata files at all and I want to render one page. And I would want probably to use collection for that.

E.x. I wanted to create static site for perun and collection didn't work because I didn't have any initial files that should be parsed and added to the meta

[bhagany]

collection now works without any input files present, as of 341ed5c