feat(doc): support multiple file entry

[bartlomieju]

This commit adds support for multiple entry points to deno doc.

Unfortunately to achieve that, I had to change the semantics of the
command to explicitly require --filter parameter for filtering symbols,
instead of treating second free argument as the filter argument.

deno doc --builtin is still supported, but cannot be mixed with
actual entrypoints.

[dsherret]

LGTM!