fix: Split out Message from Title

[epage]

In #231, Level::header was split into Level::title and Level::pre_styled_title but

• When to use each wasn't clear
• "title" suggests a stronger role within the output than it is which can be confusing
• Title contains an id even when it will go unused

This reworks things into

• Level::title() -> Title: used for the first message within a group
  • uses a built-in styling
  • can have an id
• Level::message() -> Message: used for additional text to be shown to the user
  • callers can provide their own styling
  • cannot have an id

Fixes #118

[epage]

@Muscraft not too sure what I'm missing as to why the test failure is happening. I left Message out of some places or assumed some values but, looking at how its supposed to be used, these seem correct.