Add global config with icon class option

[esl51]

PR for #25 (comment)

Now there is a global configurator, which can be expanded later. I added the iconClass parameter there to define the class that will be added to all icons.

Registering plugin with options:

import VueTablerIcons from 'vue-tabler-icons'

const app = createApp(App);

app.use(VueTablerIcons, {
    iconClass: 'my-icon-class',
});