feat: add rootNetwork CLI arg

[troggy]

Follow up for #353

/cc @vrde

Allows to do things like (needed for leapdao/leap-sandbox#54):

curl -O https://raw.githubusercontent.com/leapdao/leap-node/master/presets/leap-testnet.json
leap-node --config=./leap-testnet.json --rootNetwork=https://rinkeby.infura.io/v3/XXXXXX

[codecov]

Codecov Report

Merging #360 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
+ Coverage   95.08%   95.09%   +<.01%     
==========================================
  Files          71       71              
  Lines        1120     1121       +1     
  Branches      207      208       +1     
==========================================
+ Hits         1065     1066       +1     
  Misses         49       49              
  Partials        6        6

Impacted Files	Coverage Δ
src/utils/readConfig.js	94.73% <100%> (+0.14%)	⬆️

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