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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[gabriel-vasile]

I'm not totally against regex but it is notorious for being slow.
As far as I understand the netpbm formats,

• they all with Px, x being any digit between 1 and 7
• followed by two or tree integers
• in-between Px and each of the integer there can be any number of spaces or comments marked with #.

One problem with the current regex is that it fails for:

P2
# Created by GIMP version 2.10.30 PNM plug-in
6 # Another comment
10
255
255
255
...

I think it's worth doing a second implementation for the netpbm checks and comparing their performance. What we need for that is a function that parses the width, height numbers from the netpbm header.

[gabriel-vasile]

Something similar with this function, but stripping away the things we don't need (ex: we don't need it to return comments). We also need to give credit to the author, if we end up using our own version of that function.