Custom Theme Enabled

https://github.com/afollestad/material-dialogs/issues/100
Added some global attributes such as
```xml
<item name="md_custom_theme">true</item>
<item name="md_bg_color">#673AB7</item>
<item name="md_divider_color">#673AB7</item>
<item name="md_selector">@drawable/selector</item>
<item name="md_btn_selector">@drawable/selector</item>
```

New attribute ```md_bg_color```, ```md_divider_color```, ```md_selector```, ```md_btn_selector``` will be enabled only when ```md_custom_theme``` is ```true```.

We can also set there attribute programmatically.
```java
public Builder backgroundColor(int color)
public Builder backgroundColorRes(@ColorRes int colorRes)
public Builder dividerColor(int color) 
public Builder dividerColorRes(@ColorRes int colorRes)
public Builder selector(@DrawableRes int selector)
public Builder btnSelector(@DrawableRes int btnSelector)
```

If we don't set default for each attribute while using custom dialog, the default light or dark theme will be set as default. For example, when ```md_dark_theme``` is ```true``` and ```md_custom_theme``` is also ```true```, but ```md_bg_color``` hasn't set yet, background color might follow background color of ```md_dark_theme```.