Add support for decompression of HTTP responses

Add support for decompression of HTTP responses

If the module configuration specifies the "compression" option
blackbox_exporter will try to decompress the response using the
specified algorithm. If the response is not compressed using that
algorithm, the probe will fail.

It validates that the "Accept-Encoding" header is either absent, or that
it specifies the same algorithm as the "compression" option. If the
"Accept-Encoding" header is present but it specifies a different
algorithm, the probe will fail.

If the compression option is *not* used, probe_http_content_length and
probe_http_uncompressed_body_length will have the same value
corresponding to the original content length. If the compression option
is used and the content can be decompressed, probe_http_content_length
will report the original content length as it currently does, and
probe_http_uncompressed_body_length will report the length of the body
after decompression as expected.

Fixes #684

Signed-off-by: Marcelo E. Magallon <marcelo.magallon@grafana.com>