issue #60: adjustcolor alpha.f has no effect on svglite output

[nathanwebb]

Added in the alpha channel information to is_black(). It was only checking the red/green/blue channels, so any black with alpha was being treated as pure black.

[codecov-io]

Current coverage is 93.77%

Merging #62 into master will not affect coverage as of 1248eb7

@@            master     #62   diff @@
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  Files            6       6       
  Stmts          466     466       
  Branches         0       0       
  Methods          0       0       
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  Hit            437     437       
  Partial          0       0       
  Missed          29      29       

Review entire Coverage Diff as of 1248eb7

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[hadley]

Can you please provide a unit test?

[nathanwebb]

@hadley I've included a failing unit test for the case where colour is black with alpha of 0.5. This is addressed in PR #63. Is this right, or would you prefer me to remove that so that all tests pass?

[nathanwebb]

@hadley Have you been able to look at this PR? Do you want me to remove the failing test (which is covered by PR #63)?