Full footnote support

A few issues with footnotes, reproduced at https://github.com/jlevy/atom-flowmark/commit/a2653ff93840ce1003e003c3964484948d67def3#diff-45486205dce63c55188d5aa2d93c4877R640

Current issues:
- [x] When footnote text contains spaces, colon is erroneously escaped
- [x] Consistent handling of `<>`s around bare URLs (now always present)
- [ ] Consistent handling of spaces before and between footnote references: Remove space or newline between consecutive footnote references

Minor:
- [ ] Consider making wrapping take into account footnote references (so to prevent long lines; must also make sure this works with multiple repeated footnotes)
- [ ] Consider whether to support wrapping on footnote definitions; currently they are disabled 86e883faa08e8eff1d7fa623a7dc6163175c96d8

Issues related to footnote normalization:

- [x] Remove duplicate footnote definitions
- [x] Systematic wrapping in definitions ( 86e883faa08e8eff1d7fa623a7dc6163175c96d8)
- [x] Report on missing footnote definitions
- [x] Report on orphaned footnote definitions (and mark or remove)
- [x] Basic tests in place