offlinePageAssets -> offlineAssets, provided regardlessly

[galvez]

Change to make sure users are able to easily provide a list of extra files to precache rather than having to use cachingExtensions -- which is meant for complex use-cases. So I've removed offlinePageAssets (which only worked when offlinePage was also defined) and provided the new global offlineAsset option instead :)

[codecov]

Codecov Report

Merging #86 into develop will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           develop    #86   +/-   ##
======================================
  Coverage      100%   100%           
======================================
  Files            1      1           
  Lines            2      2           
======================================
  Hits             2      2

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a8bb3b...4128807. Read the comment docs.