Add preprocess logging

[benjeffery]

Fixes #181

Tried to add a test here but logs not coming through when run through click and didn't want to dig. Certainly gives log in the CLI.

[github-actions[bot]]

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### edges.png  ### mutations-popup.png  ### mutations.png  ### nodes.png  ### out_edges.png  ### out_mutations.png  ### out_nodes.png  ### overview.png  ### tables.png  ### trees.png 

[coveralls]

Pull Request Test Coverage Report for Build 11499907067

Details

• 12 of 14 (85.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 91.101%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tsbrowse/main.py	6	8	75.0%
<!--	Total:	12	14	85.71%	-->

Totals
Change from base Build 11498500243:	0.7%
Covered Lines:	778
Relevant Lines:	854

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💛 - Coveralls