Full footnote support

[jlevy]

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

[jlevy]

86e883faa08e8eff1d7fa623a7dc6163175c96d8