Add jinja templating; remove MarkdownTransformer (put logic in WhaleLoader); small QOL improvements

[rsyi]

This is a bit of a heavy PR, with the following changes:

• Jinja templating support, which tacks on any templates found in the ~/.whale/templates/<connection-name>.sql file by default.
• Fix: bigquery database was not correctly rendering in the markdown.
• Refactored the logic to create markdown so it all lives in WhaleLoader. Eventually, this probably should be refactored again so it cleanly lives in methods on the models, but it's fine for now.

[codecov-io]

Codecov Report

Merging #96 (5a0f26e) into master (42fc1a1) will decrease coverage by 0.03%.
The diff coverage is 78.43%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   71.86%   71.83%   -0.04%     
==========================================
  Files          31       31              
  Lines        1628     1658      +30     
==========================================
+ Hits         1170     1191      +21     
- Misses        458      467       +9     

Impacted Files	Coverage Δ
...nes/whale/extractor/bigquery_metadata_extractor.py	94.66% <ø> (ø)
...es/whale/extractor/bigquery_watermark_extractor.py	30.15% <ø> (ø)
pipelines/whale/utils/extractor_wrappers.py	76.66% <ø> (ø)
pipelines/whale/loader/whale_loader.py	59.61% <66.00%> (+1.49%)	⬆️
pipelines/whale/utils/sql.py	87.50% <87.50%> (ø)
...pelines/whale/extractor/base_bigquery_extractor.py	88.11% <100.00%> (+0.24%)	⬆️
pipelines/whale/extractor/metric_runner.py	82.05% <100.00%> (-0.62%)	⬇️
pipelines/whale/utils/markdown_delimiters.py	100.00% <100.00%> (ø)
pipelines/whale/utils/paths.py	100.00% <100.00%> (ø)
pipelines/whale/utils/task_wrappers.py	61.29% <100.00%> (+1.29%)	⬆️
... and 1 more

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