start digit only accept [0-9a-F], should accept [0-9a-Z]

[sirius0xff]

It seems rather pointless to support radix above 16 if it cannot start from digit >f.

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Also it would be pretty handy to guess radix based on the start digit:

if start in [0-9]: default_radix=10
if start in [a-F]: default_radix=16
if start in [g-Z]: default_radix=36

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Another possible improvement is to allow skipping the step. e.g. accept e::17 as e:1:17

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Thanks for the great extension.