Exclude sass partials

When using #options folder, you are likely to include sass partials (a
Sass file named with a leading underscore: from
http://sass-lang.com/guide#topic-4) in the css file list. If the
partials are not filtered out, duplicate css may be generated. For
example:

/Content/app.css.bundle
#options folder:recursive

/Content/partials/_elements.scss
form {
margin: 1em 0;
}

/Content/app.scss
@import "form";

Will result in:
form{margin: 1em 0;}
form{margin: 1em 0;}
Ensure false can be passed into options

Attempting to pass in false for any of the bundle options will result in
a string value of 'false', which is truthy. This change ensures that you
can pass in a boolean value, or a string.