(core): allow to exclude a directory from checking

Description

Allows to specify a .sheldonignore files with similar syntax as .gitignore to filter files to be sheldonized. It could be useful for files that we want to keep as is, because they belong to external sources (for example minizip sources).

adding a leading '/' means absolute path. Like in regular .gitignore, it allow to match only things at root level.

wildcard (*?...) are supported

Closes #36 (closed)

How to test it?

• create a .sheldonignore inside Sight source and add an entry like:

  /libs/io/zip/minizip/

• make a thing which should make sheldon complain (add zizi string somewhere in libs/io/zip/minizip/zip.h)
• git add ...
• sheldon -> no error, libs/io/zip/minizip/zip.h should have been ignored