Generate function to return a slice of filenames

[andreynering]

The same as AssetNames function in go-bindata.

My use case is compiling templates inside the file:

for _, filename := range filebox.FileNames() {
    data, err := filebox.ReadFile(filename)
    if err != nil {
        log.Fatal(err)
    }
    // compile template file here
}

[UnnoTed]

Hey, before merging i would like some things.

1. Make it a variable instead, as creating a new slice every time is bad.
2. Use {{exportedTitle "VariableNameHere"}} so it can be un-exported in case the user wants it to, use exported instead of exportedTitle if the variable name is all caps.
3. Provide a comment so it can be golint safe.

Ignore CI errors, i'll fix that later :blush:

edit: Keep in mind it will not show new files on the list unless you implement it to WriteFile

[andreynering]

@UnnoTed Thank you for the fast review. Updated.