Add timeout flag

[alex-treebeard]

https://github.com/treebeardtech/nbmake/issues/50

• [x] pytest option
• [x] docs

[codecov[bot]]

Codecov Report

Merging #54 (5ce6a2f) into main (ae11b73) will increase coverage by 0.83%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   91.36%   92.19%   +0.83%     
==========================================
  Files           4        4              
  Lines         139      141       +2     
==========================================
+ Hits          127      130       +3     
+ Misses         12       11       -1     

Impacted Files	Coverage Δ
src/nbmake/pytest_items.py	88.46% <ø> (ø)
src/nbmake/nb_run.py	93.44% <100.00%> (+1.77%)	:arrow_up:
src/nbmake/pytest_plugin.py	94.44% <100.00%> (+0.32%)	:arrow_up: