Add #include <cctype> and std:: qualify isspace()

[Quincunx271]

Fixes #29

It seems my editor also removed all trailing whitespace. If this is undesirable, I'll figure out how to disable that.

[codecov-io]

Codecov Report

Merging #30 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files           2        2           
  Lines        1478     1478           
=======================================
  Hits         1472     1472           
  Misses          6        6

Impacted Files	Coverage Δ
mustache.hpp	100% <100%> (ø)	:arrow_up:

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[kainjow]

Thanks!