Add an option to supply custom ASCII art.

Hello there,

I once wanted to display a custom ASCII art next to the screenFetch informations, and I ended up adding it as a "distro" in the source code, and selecting it with the `-A` option to get my custom logo.

While it was sufficient, I thought It would be slightly better to have an option to specify a custom ASCII art.

With this PR, we can give the `-a` option a path to a Bash script, defining the `startline` and `fulloutput` variables, and optionally `labelcolor` and `textcolor`.

This way, it's easy to use a custom logo without having to change the source code.

Here's an example "art file":

```bash
c1=$(getColor yellow)
labelcolor=$c1
startline=1

fulloutput=(
    ''
    "$c1"'            $$$$$$$$$$             %s'
    "$c1"'        $$$$$$$$$$$$$$$$$$        %s'
    "$c1"'      $$$$    $$$$$$    $$$$      %s'
    "$c1"'      $$  $$$$$$$$$$$$$$  $$      %s'
    "$c1"'    $$  $$    $$$$$$    $$  $$    %s'
    "$c1"'    $$  $$    $$$$$$    $$  $$    %s'
    "$c1"'    $$$$$$$$$$$$$$$$$$$$$$$$$$    %s'
    "$c1"'    $$$$$$$$$$$$$$$$$$$$$$$$$$    %s'
    "$c1"'    $$$$$$$$  $$$$$$  $$$$$$$$    %s'
    "$c1"'      $$$$$$$$      $$$$$$$$      %s'
    "$c1"'      $$$$$$$$$$$$$$$$$$$$$$      %s'
    "$c1"'        $$$$$$$$$$$$$$$$$$        %s'
    "$c1"'            $$$$$$$$$$            %s'
    "$c1"'                                  %s'
    "$c1"'                                  %s'
)
```

I don't know if the need for a custom logo option is common enough to merge this into `master`… but I still wanted to propose this change.