Supercript feature in fulltext

[kermitt2]

Add a feature in the full text model indicating if the token is a superscript.
Motivation: better learning of citation callout expressed as superscript.

Evaluated on 139,835 citation contexts and it improves the citation context resolution as follow:
from

Precision citation contexts:     81.36
Recall citation contexts:        68.73
fscore citation contexts:        74.51

to

Precision citation contexts:     81.45
Recall citation contexts:        69.58
fscore citation contexts:        75.05

Note: due to some obscure reasons, I mixed this PR with PR #572, so it will have to be merged after PR #572.

[coveralls]

Coverage decreased (-0.08%) to 37.93% when pulling d6d6a8f on supercript-feature-in-fulltext into 206583a on master.