Add ability to change label color

### Feature Request

| Q | A
|------------ | ------
| New Feature | yes
| RFC | no
| BC Break | no

#### Summary
**As far as I know there is no way to change the label color (left part).**

I think it might be a great feature to be able to add a `?label_color=FFFFFF` to further customize the badge.
With `FFFFFF`: *the value of the color in hex.*

Shields.io support this (*[See 'styles' on shields.io](https://shields.io/#styles)*):

So it would give something like this:
```php
echo $poser->generate('license', 'MIT', 'FFFFFF', '428F7E', 'plastic');
// or
$image = $poser->generate('license', 'MIT', 'FFFFFF', '428F7E', 'plastic');
```
The default color should be the default grey/gray.

---
Original: [antonkomarev/github-profile-views-counter #40](https://github.com/antonkomarev/github-profile-views-counter/issues/40).

Start at the public $poser->generate API shown in the issue and trace how badge colors are rendered. Check how the existing default grey is selected, then verify that an optional hexadecimal label color preserves the default when omitted and affects the generated badge as requested.

Written by the indexing model from the issue text.