refs https://github.com/TryGhost/Toolbox/issues/136
- `cheerio` isn't needed during the boot but it takes time and memory to
load the library
- this commit moves `cheerio` requires later into the code to when they
are needed
refs https://github.com/TryGhost/Team/issues/828
- When detecting email segments and later creating a member filter out of this data we only care about unique segments otherwise we'd be creating multiple batches with the same segment filter
refs https://github.com/TryGhost/Team/issues/828
- This is experimental segment extraction logic, more to follow. Alllows to extract arrays of email segments used in the email's HTML content
