Element imports shouldn't default to async

[madeleineostoja]

Currently Simpla imports its elements with the async, which makes their requests non-blocking. This is incorrect behaviour, and worsens the FOUC you get when Simpla's content 'pops' into place. They should instead default to be blocking, like styles and scripts in the head are, and if a user wants to have them async and handle the FOUC themselves, then they can import manually.

This also becomes redundant once #35 is shipped, as we'll be stripping out auto-imports (it will just become a matter of what we document as the recommended practice).

[madeleineostoja]

This is redundant in v2 (see 2.0-preview