Use height and width attrs when viewBox is not present

[e-pavlica]

Some of the older SVGs that our application uses were exported were exported with a height and width, but no viewBox causing this plugin to break. The "correct" way to fix this problem would be to correct the SVG sources, but that was not something I wanted to pursue.

As a non-perfect fallback for this situation, use the height and width attributes to construct a viewBox string so that compilation can continue.

I didn't add a test for this as I was reluctant to add another fixture to the repo, but can if you prefer.

I also thought about adding a console warning for SVGs missing the viewBox attr but thought that might just be annoying/heavy-handed.

[codecov]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           29        30    +1     
  Branches         6         7    +1     
=========================================
+ Hits            29        30    +1     

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)

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