Add a custom Curl probe.

Update curl output to support all timers (see below).

Add new configuration option `write_out`.
This option designate which timer information we want to get from curl output.

`write_out` can take one of the following values:

* time_namelookup
* time_redirect
* time_connect
* time_appconnect
* time_pretransfer
* time_starttransfer
* time_total

Default behaviour is the same than current Curl probe: `total - DNS`