Provide cmd.Int() and cmd.Int64() variants

[volodymyrprokopyuk]

Checklist

• Are you running the latest v3 release? The list of releases is here.
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What problem does this solve?

Currently only the var flag int = int(cmd.Int("flag")) # int64 is supported. That forces explicit conversions int(flag).

Solution description

Support all Go primitive types e.g. int, int64, float32, float64 when retrieving flag values from a command, in order avoid explicit conversions.

Describe alternatives you've considered

Otherwise, explicit conversions must be applied e.g. int(flag)

[dearchap]

@volodymyrprokopyuk There is a reason we have only 64 bit types. See #1765 , #1759

[volodymyrprokopyuk]

Reduction of the library surface area and the testing volume in expense of DX may be not the best decision.