增加allowPOS支持，即tf-idf结果仅包括指定词性的词

[citywill]

例如： $vector = JiebaAnalyse::extractTags($content, 50, ['allowPOS' => ['n']]);

[coveralls]

Coverage increased (+0.5%) to 57.911% when pulling baf446335ce245b948c4703eb548b9b6db447117 on citywill:master into a5980dc88ae98a261a0bdb1130be14d0136b373f on fukuball:master.

[codecov-io]

Codecov Report

Merging #36 into master will decrease coverage by 0.26%. The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   58.31%   58.04%   -0.27%     
==========================================
  Files           5        5              
  Lines        1353     1361       +8     
==========================================
+ Hits          789      790       +1     
- Misses        564      571       +7

Impacted Files	Coverage Δ
src/class/JiebaAnalyse.php	73.33% <22.22%> (-7.27%)	:arrow_down:

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[fukuball]

Cool, 感謝～