Added support for private network parameters

[vpulim]

Added ability to construct a Common instance using custom chain parameters. Instead of calling new Common('mainnet'), you can now call new Common(chainParams) where chainParams is an object containing chain parameters for a private network. The chainParams object must contain at least these fields networkId, genesis, hardforks and bootstrapNodes.

[coveralls]

Pull Request Test Coverage Report for Build 90

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 95.909%

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Totals
Change from base Build 79:	0.2%
Covered Lines:	127
Relevant Lines:	131

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💛 - Coveralls

[holgerd77]

Hi @vpulim, I am ok with the changes. This should be also reflected in the JSDocs, and the API documentation should be submitted in a re-generated version.

[vpulim]

Thanks @holgerd77. Updated jsdoc and regenerated docs/index.md

[holgerd77]

Ok, cool, this looks good now. Had another closer look and checked the parameters, think it's ok to leave the URL field, though things like that are generally a bit dangerous, nevertheless - I conceptualized this field anyhow for just an external reviewer informational field, so this should be ok for now.

I also had some question how hardfork parameter selection now works in a private network, but on a second thought I realized that this was irritation on my side.

I will directly prepare a subsequent release on this, as v0.5.0.