Handle boolean values for val keyword correctly

[hhaensel]

This PR fixes #57 by allowing supply of Bool values,

julia> radio("Bar plot", :show_bar, val = true)
"<q-radio :val=\"true\" label=\"Bar plot\" v-model=\"show_bar\"></q-radio>"

With this solution you are still allowed to write

julia> radio("Bar plot", :show_bar, val = "true")
"<q-radio val=\"true\" label=\"Bar plot\" v-model=\"show_bar\"></q-radio>"

which will attach the string "true"