Pre-compile some date regexes

[J08nY]

Precompiling some regexes in date.py offers some speedup, as compiling static regexes on every function call is pointless.

Before:

After:

Test file:

import dateparser

for i in range(1000):
    date = dateparser.parse(" " * i + "12.18.2015")

[codecov-io]

Codecov Report

Merging #293 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   94.42%   94.44%   +0.01%     
==========================================
  Files          34       34              
  Lines        2960     2969       +9     
==========================================
+ Hits         2795     2804       +9     
  Misses        165      165

Impacted Files	Coverage Δ
dateparser/date.py	93.13% <100%> (+0.31%)	⬆️

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