Introduce expansion for file extensions

[EmilyGraceSeville7cf]

To write something like: {/?file image: rose.(jpg, jpeg)} to be expanded to {/?file image: rose.jpg rose.jpeg} to resemble alternative's syntax in code description.

Notes

• parenthesis should not be escaped
• spacing after dot doesn't matter (the last dot is considered as filename and extension delimiter)
• spacing between extensions doesn't matter
• no extension can have escaped space in it

[EmilyGraceSeville7cf]

Probably I should consider incorporating Go templates.