make default html show fullmode

[johnnychen94]

Resolves the comment in #51 (comment)

I still think that the downscaling done by the HTML show method can be confusing, because it happens secretly, and it is not possible to control this behaviour.

@fonsp Thanks for catching this! I believe I made a mistake in #50 that assumes IOContext downsamples image by default -- but we should not.

For the ImageShow.preview(img) thing, people might just refer to ImageBase.restrict or ImageTransformations.imresize.

[codecov]

Codecov Report

Merging #53 (0eb0a32) into master (ce60b0a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
=======================================
  Coverage   87.28%   87.28%           
=======================================
  Files           6        6           
  Lines         236      236           
=======================================
  Hits          206      206           
  Misses         30       30           

Impacted Files	Coverage Δ
src/showmime.jl	95.55% <100.00%> (ø)

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[fonsp]

Yayyy thanks Johnny!!