Support more units and ignore case in file-size config options

[jameslamb]

fixes #274

Some configuration options, like --max-allowed-size-compressed, allow you to change file-size-based thresholds that influence check results.

These are currently evaluated in "bibytes", but are not correctly document. e.g. "1M" = "1 mebibyte" = 1024^2 bytes, but pydistcheck docs say that "1M" = "1 megabyte", which is 1,000,000 bytes.

This PR:

• fixes those docs
• adds support for using the clearer form like Mi or Gi
• adds support for really specifying sizes in other units, like "MB" for megabytes
• ignores case, so that now these would all evaluate to the same number of bytes: 5KB, 5kB, 5Kb, 5kb

Why have the single-letter aliases like M support e.g. mebibytes instead of megabytes

To avoid breaking any existing pydistcheck uses that have been relying on that behavior.

And because I couldn't find any reference suggesting a preferred way to interpret the unit M.