Add support for custom footer text

[thoughts3rased]

In some cases, users may want to display custom text in the footer of the embed to give extra detail. This could be the time at which the embed was requested, the author of the original interaction or a custom message to users.

With this change, I've added an additional argument, customFooterText to allow a string to be passed containing the text that the user wants to display next to the page count, separated by a hyphon.

Example:
Say the user wants the time of the embed's request to be set as the footer. ("Requested at 12:00PM")

When passed through, the footer text would appear like this:
Page X/Y - Requested at 12:00PM

I hope this adds some functionality to this already extremely useful module.

[thoughts3rased]

It should be important to note that this argument that has been added is optional, and defaults to an empty string to not cause problems with setting the footer later on down the line.

[ryzyx]

Not sure if many people require this