Custom request timeout setting

[iamdefinitelyahuman]

What I did

Allow custom timeout value for networks.

How I did it

• Add a timeout field for each network within network-config.yaml - if not present, the default value is 30 seconds
• Set the default timeout for mainnet-fork as 120 seconds.

How to verify it

Run tests.

[codecov-io]

Codecov Report

Merging #469 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #469   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files          56       56           
  Lines        6071     6071           
  Branches     1336     1336           
=======================================
  Hits         5357     5357           
  Misses        494      494           
  Partials      220      220           

Impacted Files	Coverage Δ
brownie/_cli/networks.py	77.31% <100.00%> (ø)
brownie/network/main.py	87.17% <100.00%> (ø)
brownie/network/web3.py	97.14% <100.00%> (ø)

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