allow lists to be passed to bind_draws

[mjskay]

Summary

This PR closes #253 by allowing lists of draws objects to be passed as the first argument to bind_draws().

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Merging #259 (a655ef4) into master (c95d6de) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #259   +/-   ##
=======================================
  Coverage   95.05%   95.05%           
=======================================
  Files          42       42           
  Lines        2931     2932    +1     
=======================================
+ Hits         2786     2787    +1     
  Misses        145      145           

Impacted Files	Coverage Δ
R/bind_draws.R	96.96% <100.00%> (+0.01%)	⬆️

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[paul-buerkner]

Thank you! Looks good. Merging now.