Transport options API

[offa]

The configuration string is too limited for certain features (see discussion) and an API for different types of options is needed.

Related:

• Read timeout on InfluxDB-cxx #171
• Add option to allow Curl to not verify self signed certs #114

[offa]

A possible implementation could look like this (using some of the related issues for demonstration):

auto db = influxdb::http("http://localhost:8086?db=test")
    .setProxy(…)
    .setTimeout(10_s)
    .setVerifyCert(false)
    .connect();

This would enable settings for Http transport only and not alter the Transport interface for all classes. What do you think?