icons about vuejs-datatable

[retrogradeMT]

Ok, I added them separately, so this isn't an issue.

[GerkinDev]

@pstephan1187 I did not find any clear documentation (other than declaring a new table type) that allows to use different options for icons. Maybe adding a prop on the datatable component to get settings overrides may be useful.

Maybe related to #35 .

If you think it is relevant, I'll open a new issue for this.

[pstephan1187]

Currently, the only way to change the icons is via the sorting classes in the settings object when defining new tables. I am open to being able to override settings other ways and also defining sorting icons other ways as well.