feat(styling): keep Nuxt style when css=false

[farnabaz]

When project are mainly targeted for AMP, removing styles creates bad DX. On the pure AMP project developers should should be able to define styles in SFC.
This PR creates a solution to disable style removal for the pure amp project.

close #215 #216

[codecov]

Codecov Report

Merging #218 (480693d) into master (c71406b) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
- Coverage   67.79%   67.77%   -0.02%     
==========================================
  Files           5        5              
  Lines         177      180       +3     
  Branches       49       50       +1     
==========================================
+ Hits          120      122       +2     
- Misses         41       42       +1     
  Partials       16       16              

Impacted Files	Coverage Δ
lib/module.js	67.81% <66.66%> (-0.05%)	⬇️

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