Allow fallback to width/height for symbol viewBox

Introduces a new recognized option called 'whviewbox'.

Ordinarily, each generated symbol in the output file only gets a viewBox
if the original source SVG had one. When enabled, whviewbox makes it so
that having both a width and height on the original SVG will allow a
viewBox attribute to generated automatically.
Documentation for whviewbox flag

Adds a new entry after the existing symbol option explaining how
whviewbox works and how it differs from the default behavior, including
a short example.
Unit test for whviewbox piggy-backing on cleanup

Given that the cleanup.svg fixture already typifies my use case (i.e. a
source SVG whose width/height is set, but not viewBox), this piggy-backs
on that by adding the 'whviewbox' flag and seeing if the expected output
contains the proper added viewBox.
Rename whviewbox to inheritviewbox

The latter name is more intuitive, as 'whviewbox' is rather low-level
for what it is doing.
Merge pull request #66 from corpulentcoffee/whviewbox

Optionally derive symbol viewBox from its source SVG width/height