grok : allow dash "-" in alias names

[vjeantet]

The following configuration grokfails because of LOGLEVEL log-level alias, when alias is loglevel it works well

input

2016-07-11T23:56:42.000+00:00 INFO [MySecretApp.com.Transaction.Manager]:Starting transaction for session -464410bf-37bf-475a-afc0-498e0199f008

filter

grok {
   match => { "message" => "%{TIMESTAMP_ISO8601:timestamp} %{LOGLEVEL:log-level}" }
 }

grok processor should accept dashbased alias