Make PDF limit size optional

[MRJCrunch]

After this PR #2146 width/height limit is hardcoded, but according to new PDF specs (page 1128) page size was increased.

Beginning with PDF 1.6, the size of the default user space unit may be set with the UserUnit entry of the page dictionary. Acrobat 7.0 supports a maximum UserUnit value of 75,000, which gives a maximum page dimension of 15,000,000 inches (14,400 * 75,000 * 1⁄72). The minimum UserUnit value is 1.0 (the default).

I have several cases when I need to insert a big image (20k pixels) inside the pdf file without compression, but I get a warning after calling constructor and in the output file image is cut from the right side.

new JSPDF('p', 'px', [20000, 30000]);

Is it possible to make this limitation optional?