Add context Writef feature function

[asyslinux]

Please provide enough information so that others can review your pull request:

Request to add additional function ctx.Writef(f string, a ...interface{}) . Tests included.

Explain the details for making this change. What existing problem does the pull request solve?

The ability to use sending a response with data as in other web frameworks that have this feature.

Example:

world := "World!"
_, err = ctx.Writef("Hello, %s\n", world)
if err != nil {
    log.Fatal(err)
}

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[ReneWerner87]

thx for the feature PR

• please add a documentation in our doc repository
  https://github.com/gofiber/docs/blob/master/api/ctx.md

[asyslinux]

gofiber/docs#250 - documentation

[ReneWerner87]

Thx

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