Add regex for line comparison

[cns-solutions-admin]

This solves #242 by adding a new optional regex to compare the lines other than the first for multiline headers.

[cns-solutions-admin]

Also converts a inline header style's name to lower case, otherwise the comparison (and the example in the docs) fails.

[mathieucarbou]

Hello, Why changing the header definition didn't work for your use case ?

[cns-solutions-admin]

Changing the header definition did not work, because the detection of header lines was orginally done by comparing lines with startsWith(beforeEachLine.rtrim()) the old " * something" does not start with "**".

By introducing a (optional) RegEx for the header lines, this and most other use cases can be covered. It also fits with the current configuration, where we already have RegExs for start and end line.