Make a simpler command line usage

... as Unite does. For exmaple:

```
EightHeader -inline -decor=['l','decor','r']
```

Do a Dict of settings:

```
s:settings = {
\ '_': 'factory defaults',
\ }

g:settings = {
\ '_': 'user defaults',
\ 'custom_theme': 'settings of custom theme',
\ }
```

To use the theme settings:

```
EightHeader -theme=custom_theme
call EightHeader('custom_theme')
```
