ENH: Add bids.ext namespace package for subpackages

[effigies]

This will enable #814.

[codecov]

Codecov Report

Merging #820 (39d67ed) into master (f422263) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   85.93%   85.91%   -0.03%     
==========================================
  Files          34       35       +1     
  Lines        3883     3884       +1     
  Branches      998      998              
==========================================
  Hits         3337     3337              
- Misses        352      354       +2     
+ Partials      194      193       -1     

Impacted Files	Coverage Δ
bids/ext/__init__.py	100.00% <100.00%> (ø)
bids/__main__.py	0.00% <0.00%> (-33.34%)	⬇️

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