Allow for customizing the fill character (default to space)

We may want to introduce some mechanism to allow this to default to the space character, but since Rust doesn't have default parameters, this might involve making a struct and forcing the user to use that, which we do not want. We want the user to be able to call the methods directly on strings, so it might just be best to force them to supply the fill char on each function call.

Research this a bit, ask Rust Discord server

We might just want to double each method in the trait impl and rename the copy as "_with_fill_character" - see [branch](https://github.com/JosephTLyons/text_align_rust/tree/Add-fill-character-option)