Release v1.4.0

Release Notes

• Fix TravisCI build.
• Add autoload cookies for obsolete markers #35.
• Added own current page implementation to unify doc-view=/=pdf-tools #31.
• Make sorting more robust (don't error when there are no headings to sort).
• Add more tests.
• Refactor PDF/notes synchronization functions #23.
• Allow subheadings to be added to notes. Make interleave-sync-pdf-page-current to work from subheadings #24.
• Make slit direction customizable. See interleave-split-direction #32.
• Add options to enlarge/shrink the notes buffer.
• Allow multi-pdf notes to start on any heading level #33.
• Make narrowing optional. See interleave-disable-narrowing #34.
• Various minor improvements (smaller bugs and refactorings).