Allow for custom page sizes on image prints

[akberenz]

Address the issue with custom sizing for images on larger paper sizes not included in the printer driver brought up in #1413.
This is an alternative to #1410 and moves the custom sizing option to the config level instead of in the data.options level.

Adds the custom boolean flag under config.size to allow bypassing the job.validatePage check on image prints, potentially allowing for printing on ages larger than specified in the driver.

Closes #1413
Supersedes #1410

[tresf]

@antoineveldhoven can you test this PR?

var config = qz.configs.create("printer name", { size: { custom: true, width: nnn, height: nnn });
//                                                       ^--- HERE

[tresf]

@antoineveldhoven Still hoping this makes the upcoming 2.2.6 release, any luck testing on your end?