ENH: Add a command line interface

[oesteban]

Resolves: #46.
Supersedes: #95.

[codecov]

Codecov Report

Attention: Patch coverage is 69.35484% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 84.28%. Comparing base (ead040a) to head (6f27171).

Files	Patch %	Lines
templateflow/cli.py	68.85%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   87.33%   84.28%   -3.06%     
==========================================
  Files           6        7       +1     
  Lines         308      369      +61     
==========================================
+ Hits          269      311      +42     
- Misses         39       58      +19     

Flag	Coverage Δ
api	84.28% <69.35%> (-3.06%)	⬇️
config	49.86% <1.61%> (-9.88%)	⬇️
datalad	78.59% <69.35%> (-1.93%)	⬇️
dls3	53.38% <1.61%> (-10.58%)	⬇️
s3	55.28% <1.61%> (-10.95%)	⬇️

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[oesteban]

It's a first draft - let's add tests and improve from here.