Adding nested tags for slice of strings

[HARDY8118]

As pointed out in the issue #37 (faker slice of urls don't work), struct fields do not support faker tags for the underlying field data type in case of slices. Taking a subsection of the mentioned issue

type Entity struct {
    ...
    Uris []string        `faker:"slice_len=3, url"`  // makes all structure empty. I just need slice with 3 urls
    ...
}

In the above code taken from issue #37 the Uris slice will not take into account the url tag.

For the mentioned issue, I suggest an extension to the faker struct tags syntax to allow nested tags with following syntax:

type SomeType struct {
    ...
    SomeSliceOfStrings []string `faker:"<tags for slice>, [<tags for string>]"`
    ...
}
// Example
type Emails struct {
    EmailList []string `faker:"slice_len=3, [email]"`
}

The PR is still a work in progress but I would like to hear more and discuss on the idea of using nested tags.