add func `Atoi` to convert string base 10

[superqtqt]

add func Atoi to convert string base 10, example : "0890" -> 890

#https://github.com/spf13/cast/issues/147

[CLAassistant]

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[yveshield]

This is a really bad idea. Since the beginning of the C language, there has been a convention of starting with "0" for octal numbers and "0x" for hexadecimal numbers. Why should we change this convention? If you want to remove the leading 0, do it before calling cast.