Add alwaysUseActiveTerminal setting #123

[andycraig]

Solves #123

What problem did you solve?

Currently, only command R: Run Selection/Line in Active Terminal works for terminals other than the R Interactive terminal. This PR adds a setting r.alwaysUseActiveTerminal (false by default). When this setting is enabled, all commands (R: Run Source, R: Preview Dataframe, R: Knit Rmd etc.) are sent to the active terminal. This allows SSH users etc. to access the full functionality of this extension.

Screenshot

Before this PR, both R: Run Selection/Line and R: Preview Dataframe would have opened a new R Interactive terminal.

Notes

I needed to create the function chooseTerminalAndSendText outside activate in extension.ts so that it could be exported for use in preview.ts. I decided to put it in rTerminal.ts. I moved the associated terminal choosing/sending functions in there, because they couldn't be exported while inside activate either. I created a separate commit just for moving these functions, to make it easier to compare the changes.

Command R: Run Selection/Line in Active Terminal is probably redundant now, but I left it in there so that this PR doesn't remove any functionality.

[Ikuyadeu]

@andycraig Thank you for a lot of work!
Thanks to your perfect work, I could not find any problem and other tasks that I should do (e.g. Refactoring, Running the tslint, Make a Readme).
So, I will publish this version soon.

[andycraig]

@Ikuyadeu Happy to hear that :) Thank you!