Added the ability to modify alt text within convert_image function

[mattl1598]

Fix for issue #103 which I opened last year, forgot about and found my own issue when trying do the same thing again in another project.

• Modified the document alt text grabbing to remove new line characters.
• Added check for replacement alt text.
• Added a test to verify the alt text changing works.

[mwilliamson]

This is now supported in 1.5.0.