Major - Configurations

This allows users of go-debug to define how and what to debug with delve. All details on how to add configurations to your atom can be read in the README.
Fixes several issues: #49, #31, #57, #59, #61, #63

Minor

• it is now made out of etch (a UI lib from atom) instead of react
• use version 2 of delve's RPC connection
• better integration with go-plus: instead of having a separate output panel on the bottom, go-debug now adds a tab to the panel provided by go-plus

• only the "panel initial visible" setting now determines whether the panel is visible on startup and completely ignores the previous visible state (#54)

• allow a custom dlv executable to be used using the new "dlv path" setting in go-debug (#50)
• disabling the go-debug package while debugging does not fail with an exception anymore (#47)
• editor tracking is now more robust and won't fail on path changes (rename, move) of a file (#48, #21)

Detailed changes: https://github.com/lloiser/go-debug/compare/v0.1.20...v0.1.21

• Change the default for panelInitialVisible from true to false to prevent (by default) the debug window from being visible at all times

Detailed changes: https://github.com/lloiser/go-debug/compare/v0.1.19...v0.1.20