More options for image size

[steffansluis]

Why is this a discrete variable and not just a pixel value? In any case, this very simple addition allows for bigger images without breaking anything. Apologies for not following the contributing guidelines. Due to the simple nature of the change I did not feel it would add in any way to the value of the change.

[neocotic]

The reason is because the aim of the first release (and the repo as a whole) was to create a usable API wrapper on top of tz's jsqrencode project. Since then I've just been working on improving the API, without really changing the guts.

Although increasing the number of available sizes seems like a good fix, there are some scaling problems that become more apparent at larger sizes. I raised #2 long ago to track this, but I've not had time to. The problem is that the margin/padding increases as the size increases causing the actual QR code to not fill the area.

For this reason, I'm hesitant to support larger sizes until that issue's fixed, otherwise it will no doubt result in duplicate issues being raised. However, once this has been done, I will probably support pixel dimensions to be specified as well as scaling.