Customizable html template style

[kkkkan]

This is adding feature pull request.

Add customizable style section of template html and output html's style is customizable . For example , if in app's build.gradle

licenseTools {
    customStyles = "body{
    background-color: pink;
    }"
}

and do generateLicensePage task , generated licenses.html's background color is pink.

If you do not set customStyles configuration in app's build.gradle and do generateLicensePage task , generated licenses.html is same in generated made by com.cookpad.android.plugin.license-tools v1.2.8.