@luisestrada
Done. (Released. v1.4.2)
A new config value, relativeNamedDayOptions, is introduced. Now People can modify the option generally.
However, for your specific purpose (en X días instead of dentro de X días), further step is needed.
Only the Mexican locale supports en X días format with relativeNamedDayOptions: {style : 'short' }, other Spanish-locales don’t.
So to get en X días, you have to use locale: 'es-MX' regardless of which country you live.
locale: "es-MX",
relativeNamedDayOptions: {
style: "short",
},

If you are living in Mexico, there would be no issue. But in other countries where the week starts on Monday, 2 more configuration is needed.
locale: "es-MX",
relativeNamedDayOptions: {
style: "short",
},
firstDayOfWeek: 1,
minimalDaysOfNewYear: 4,

Of course, if you don’t care dentro de 3 días, You can use any locale under which you live.
locale: "es-ES",
