Support arbitrary headers with HTTP datasources

[hairyhenderson]

Fixes #113

Adds new a command-line option --datasource-header/-H, which allows sending custom headers when accessing HTTP[S] datasources. For example:

$ gomplate -d foo=https://httpbin.org/get -H 'foo=Foo: bar' -i '{{(datasource "foo").headers.Foo}}'
bar

This can be used e.g. for authentication, by setting the Authorization or related headers...

Signed-off-by: Dave Henderson dhenderson@gmail.com