parser panic on wrong type conversion

[cocap10]

Flaeg panics if types in custom parsers don't match each other
In func (s *SliceStrings) SetValue(val interface{})

panic: interface conversion: interface is Namespaces, not []string

Example :

// Namespaces holds kubernetes namespaces
type Namespaces []string
//...
f.AddParser(reflect.TypeOf(Namespaces{}), &flaeg.SliceStrings{})

[cocap10]

Try to fix it :
AddParser func(typ reflect.Type, parser Parser) error
could return an error if typ and parser don't match each other
see https://github.com/containous/flaeg/tree/fix-parser-type-conversion