Add lenses for config

[Martoon-00]

Clarification and motivation

It would be very convenient if the configuration could be tuned up with lenses and it is possible to write e.g.

int = mkInt $ defaultInterpolatorOptions &~ do
  defaultSwitchesOptionsL . defSpacesTrimmingL ?= True

Note that we can be lens-compatible without actually depending on lens package and rather declaring our own Lens' type alias.

Acceptance criteria

• Lenses for every field appearing in config-related types are added.
• The library does not rely on lens package, nor on microlens package.
• The tutorial mentions a use case with these lenses.