implement start & end delimiters

[brenol]

This PR implements the possibility to pass delimiters as options to the interpolator.

This way, those who desire can use interpol to work with JSON strings.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@5accad8). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master     #1   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?      3           
  Lines             ?    132           
  Branches          ?      0           
=======================================
  Hits              ?    132           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
options.go	100% <ø> (ø)
interpol.go	100% <100%> (ø)

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