Add TrimOption

[tmontaigu]

Adds a ReadingOption that enables to choose how character fields should be trimmed.

Addresses #56

- [ ] Put the encoding as a reading option ?

• [x] add builder pattern + non_exhaustive to prevent api breaks when adding new options later
• [x] docs - [ ] Should options be per field ? -> more complex to do

[tmontaigu]

Actually, I think the better thing to do is to only trim the end and not have an option for now