Added support for overriding color and name for level along with custom levels.

[iamsumit]

This PR adds the support for customizing the custom log level name and color along with option to override the color of existing levels.

resolves #61

[lmittmann]

I appreciate the effort, but am not convinced by the API. slog.Level is an alias for int - why use a map from slog.Level to its string representation?

[iamsumit]

why use a map from slog.Level to its string representation?

I am not a go expert, and I know that this is not an elegant solution but I don't understand your doubt here? A map key can be any type that is comparable, be it string or integer.

[lmittmann]

A map lookup is expensive. The default slog logic is to print LEVEL+N e.g. INFO+1 for "unmapped" level values, but it is not efficient to build a map that maps all values of int to a string.