Add request headers to traits available to segment rules

[dabeeeenster]

It would be nice to be able to write segment rules using HTTP request headers as trait values. E.g a browser might send the following HTTP Request header through:

accept-language: en-GB,en-US;q=0.9,en;q=0.8

We could then enable users to add a segment rule based on this data as a transient trait, with the name header:accept-language and the value en-GB,en-US;q=0.9,en;q=0.8