doc: add ReST doc generation

[shahidhk]

Added GenReST commands to generate docs in ReStructured Text format.

This is exactly similar to GenMan and GenMd commands. To deal with document processors like Sphinx, which can cross link documents using :ref: tags, linkHandler function can be used.

[CLAassistant]

All committers have signed the CLA.

[n10v]

LGTM
/cc @eparis

[shahidhk]

@BoGeM @eparis Any update on this? Thanks

[n10v]

Please add a reference to docs in README as it shown here.

[shahidhk]

@BoGeM Done. I have also combined all three document generation headings into one. Hope that's fine.

[n10v]

Please resolve conflicts and add link to Table of Contents.

[shahidhk]

@BoGeM sorry about that. I have merged master and fixed conflicts, also added to ToC

[shahidhk]

Looks like some tests are failing on circleci:

--- FAIL: TestNormConsistent (0.00s)
	cobra_test.go:1253: Normalizing flag names should not result in duplicate flags

happened on my local too. But passed on travis.

[n10v]

happened on my local too

Update pflag on your machine: go get -u github.com/spf13/pflag

[n10v]

Thank you for your PR!

[shahidhk]

Thanks @BoGeM for guiding me through this.