Add command to extract method out of selected code

[ramya-rao-a]

Related to a feature ask in #275
This commit adds a command to the command palette that can be used to extract a method out of selected code and replace the selected code with a call to the extracted method using godoctor. Read more about godoctor at http://gorefactor.org/

godoctor does have limitations in the extract method feature.

Code containing return statements, defer statements, or anonymous functions cannot be extracted

Do provide feedback if godoctor seems like a wise choice for the extract method feature

[msftclas]

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[ramya-rao-a]

Closing this PR in favor of #2139