New API for library configuration

This is the first step into providing (more) configuration options in nDPI.

The idea is to have a simple way to configure (most of) nDPI: only one
function (`ndpi_set_config()`) to set any configuration parameters
(in the present or on in the future) and we try to keep this function
prototype as agnostic as possible.

You can configure the library:
* via API, using `ndpi_set_config()`
* via a configuration file, in a text format

This way, anytime we need to add a new configuration parameter:
* we don't need to add two public functions (a getter and a setter)
* we don't break API/ABI compatibility of the library; even changing
the parameter type (from integer to a list of integer, for example)
doesn't break the compatibility.

The complete list of configuration options is provided in
`doc/configuration_parameters.md`.

As a first example, two configuration knobs are provided:
* the ability to enable/disable the extraction of the sha1 fingerprint of
the TLS certificates.
* the upper limit on the number of packets per flow that will be subject
to inspection

tests/cfgs/disable_metadata/config.txt

@@ -0,0 +1 @@
+--cfg=tls,metadata.sha1_fingerprint.enable,0