feature: decode from string

[kamilsk]

Motivation: there is no way to operate features (enable/disable them externally).

Draft

header := "X-Features: feature-a=true, feature-b=false"

features.Unmarshal(header)

PoC

vals := req.Header.Get(header.Features)

features.Apply(header)
if features.FindByName().Enabled {
  // do something
}

Related to octolab/protocol/issues/10.

To do:

• Find way to decode "multi-value" header. Is ; the separator?