feat: Add `length` support

[thofma]

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Adds a keyword argument length=ex to allow for specifying the length of the iterator.

Closes #41

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 59.52381% with 17 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@3f7d845). Learn more about missing BASE report. Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/macro.jl	59.52%	17 Missing :warning:

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #102   +/-   ##
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  Coverage          ?   79.78%           
=========================================
  Files             ?        6           
  Lines             ?      455           
  Branches          ?        0           
=========================================
  Hits              ?      363           
  Misses            ?       92           
  Partials          ?        0           

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[Krastanov]

Thank you! This looks great. The failure on Julia nightly is due to how the @doc test is written (it is a bit fragile). I will make an issue about it and merge this.