Add support for quantile and quantileIf functions

[aronbierbaum]

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-sqlalchemy.readthedocs.io/en/latest/development.html.

[coveralls]

coverage: 85.986% (+0.04%) from 85.943%
when pulling 0cea38d on aronbierbaum:add_quantile
into ac9442a on xzkostyan:master.

[aronbierbaum]

@xzkostyan Could you review the addition of custom quantile and quantileIf functions. The main issue I ran into is that using the default function proxy does not support generating quantile(0.75)(column). In order to support the function I added a custom function that compiles quantile(0.75, column) to quantile(0.75)(column).