add support for arbitrary bit depths (<= 32)

[chrstphrbrns]

Fixes #58

To test various bit depths, this command will be useful:

% convert image.tif -depth N [-colorspace gray] [-interlace plane] [-compress zip/lzw/rle/none] out.tif

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (79d5508) 92.76% compared to head (bc0a4d2) 92.89%.

Files	Patch %	Lines
src/ifds.jl	95.68%	5 Missing ⚠️
src/layout.jl	86.84%	5 Missing ⚠️
src/compression.jl	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   92.76%   92.89%   +0.12%     
==========================================
  Files          13       13              
  Lines         982     1069      +87     
==========================================
+ Hits          911      993      +82     
- Misses         71       76       +5     

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[tlnagy]

Awesome work, some minor points