cmd/addchain: search options to modify costs

[mmcloughlin]

Doubling (squaring) is typically cheaper than addition (multiplication). This
PR adds options to the search subcommand to score addition chains based on a
weighted sum.

Closes #55
Updates #117

[codecov-commenter]

Codecov Report

Merging #120 (32fccb5) into master (af1add4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #120   +/-   ##
=======================================
  Coverage   73.05%   73.05%           
=======================================
  Files          30       30           
  Lines        1874     1874           
=======================================
  Hits         1369     1369           
  Misses        445      445           
  Partials       60       60           

Flag	Coverage Δ
stress	?
unittests	73.05% <ø> (ø)

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[armfazh]

These changes look good, a validation that the costs are non-negative could be necessary.