Add "baseSize" setting to control the font-size on the .icon class

I was having some problems in IE with the hard-coded 10px font-size. Possibly related to a minimum font-size allowed in the browser, but I eventually gave up trying to figure it out and just modified gulp-svn-sprites to allow for a different font-size to be set. By changing the font-size to 16px, all my problems went away. This will also be useful in the case that there will be text in the same HTML element as the SVG icon.

### Usage:
```js
gulp.task('sprites', function () {
    return gulp.src('assets/svg/*.svg')
        .pipe(svgSprite({
            baseSize: 16
        }))
        .pipe(gulp.dest("assets"));
});
```

Will output a CSS file starting:
```css
.icon {
    font-size: 16px;
}
```

And will adjust all the em values in the CSS file as appropriate.