Adding support for stdin: scheme for datasources

[hairyhenderson]

Fixes #230

Adds support for a stdin: scheme for datasources. For now, MIME type is specified by setting a bogus filename and the extension is used. If no parsing is required (i.e. if the data is being included verbatim with the include function), just stdin: is enough.

$ echo 'foo: bar' | gomplate -i '{{(ds "data").foo}}' -d data=stdin:///foo.yaml 
bar
$ echo 'foo' | gomplate -i '{{ include "data" }}' -d data=stdin:
foo

Signed-off-by: Dave Henderson [email protected]