enhancement / templateFunc `firstof`

added template function `firstof` to return the first non-'null' argument
per the `coalesce` in https://github.com/Jetvp/revel/commit/b67c19c0aaebfbd818e6ffc0a597ac48bcc30397

this should help deal with issues: #221 #451

example:

`{{firstof $field.Flash $field.Value}}`
`{{first of .User .Guest}}`

Not the params thing though