Add FilterCounter

[e9t]

Inherit Counter from collections, and apply filters.

Some filter examples:

• Minimum length of terms (minsyl)
• (General) Stopwords
• Include/exclude specific (POS or other kind of) tags

Usage example:

from konlpy.corpus import kolaw
from konlpy.tag import Hannanum
from konlpy.utils import FilterCounter

doc = kolaw.open('constitution.txt').read()
pos = Hannanum().pos(doc)
cnt = FilterCounter(pos, minsyl=2, stopwords=True, ta

cnt = FilterCounter(pos, minsyl=2, stopwords=True, postags="N|V|EF|^XPV|^SF")
cnt.most_common(20)

[e9t]

Filter과 counter는 따로 있는게 좋겠다는 의견이 있었습니다.
(Sequence를 보존하고 싶은 경우가 있을 수 있음.)