Support reading query param and header values from a file

[ducaale]

This PR adds variants of : and == with @ postfix for reading values from a file.

# send a query param with a value read from temp.txt
$ xh httpbin.org/get test==@~/temp.txt

# send a header with a value read from temp.txt
$ xh httpbin.org/get x-api-key:@~/temp.txt

Also, see httpie/cli#1225.

Ticks another item from #4.

[blyxxyz]

Works great!