Add an option to control the default size of the image

[yutannihilation]

Fix #21

Internal

Currently, conflr sets the height to 400 on all images that don't have height attribute; if an image has its own heights and width, those are respected by the change of #7.

This PR changes to set width to 600 on on all images that don't have width attribute. Probably, this changes the visual appearance, but setting width is in line with what the Confluence's editor does.

User visible changes

This PR adds a new argument image_size_default to confl_post_page() and confl_update_page(). When this is NULL, no width or height will be set, which means the images will be displayed as the original sizes.

But the parameter is not directly exposed to the addin. Instead, a simple checkbox is added for simplicity. If this is checked, image_size_default = NULL is used.

TODO

• Update README in another PR