mark covertool as project_plugin

[TheGeorge]

coverall should not be a transitive dependency for proper, and only
be respected when developing proper itself (specifically for CI runs)

see http://rebar3.org/docs/configuration/plugins/#project-plugins-and-overriding-commands

[codecov-commenter]

Codecov Report

Merging #283 (9b6f232) into master (a17e23e) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
- Coverage   85.32%   85.28%   -0.05%     
==========================================
  Files          14       14              
  Lines        4586     4586              
==========================================
- Hits         3913     3911       -2     
- Misses        673      675       +2     

Impacted Files	Coverage Δ
src/proper_erlang_abstract_code.erl	93.57% <0.00%> (-0.34%)	⬇️
src/proper_types.erl	94.77% <0.00%> (ø)
src/proper_gen.erl	87.19% <0.00%> (+0.49%)	⬆️
src/proper_arith.erl	92.70% <0.00%> (+1.04%)	⬆️

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