Fix `NUMBER_PATTERN` regex for parsing SVG viewboxes

[jams2]

Fixes #127

The NUMBER_PATTERN in the SVG viewbox regular expression was missing the optional + or - in the final integer production.

The definition for number, as used in SVG attributes, can be found here:

• https://www.w3.org/TR/SVG11/types.html#DataTypeNumber
• https://www.w3.org/TR/2013/WD-SVG2-20130409/types.html#DataTypeNumber