Added support of netpbm files

[ysyrota]

Added support of netpbm files (PAM, PFM, PBM, PGM, and PPM) and Regular Expression based detector.

[codecov-commenter]

Codecov Report

Merging #283 (c6a596b) into master (fe5b52f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #283   +/-   ##
=======================================
  Coverage   94.96%   94.96%           
=======================================
  Files           3        3           
  Lines         159      159           
=======================================
  Hits          151      151           
  Misses          6        6           
  Partials        2        2           

Impacted Files	Coverage Δ
tree.go	50.00% <ø> (ø)

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