Add oll-base.ily file, to be included by all files

This file provides common functionality etc.
For now this is:

- initialization of all header fields
- markup commands for openlilylib documentation
oll-example: Improve usage example include file

define "includeName" as the path to be included
The module author only defines #(moduleName "...") with the
file's basename.

Add markup command for consistent documentation formatting
Provide initial \bookTitleMarkup for usage examples

Makes use of a number of header fields, including the
filename to be included.
git-commands: Add \gitIsCleanMarkup

markup command to return an arbitrary string depending on the
git status of the repository
git-commands: Update header fields
git-commands: Update Usage Example file

TODO: Make \gitDiff print on multiple pages.