Fix plot viewer

[renkun-ken]

What problem did you solve?

Partially fixes #364.

This PR:

• Restores the previous defaults of plot and View position (my negligence in Session watcher options #359) and fixes the behavior of using vsc.plot that could be a string.
• Adds up a section of available functions and options to README.

I'll look into if I could make option vsc.plot effect immediately in another issue, which could be tricky.

We might need a quick release for this so that user could get the previous behavior of viewing both plot and View() in ViewColumn.Two.

(If you do not have screenshot) How can I check this pull request?

1. Write options(vsc.plot = "Two") in ~/.Rprofile.
2. Start a new session and run plot(rnorm(100))
3. The plot file shows up at ViewColumn.Two.
4. Run options(vsc.plot = "Active") and .vsc.attach().
5. Run plot(rnorm(100)) again.
6. The plot file shows up at ViewColumn.Active.

[renkun-ken]

I'm about to add some documentation in the README about the currently available options in this PR.

[Ikuyadeu]

@renkun-ken Good, you can merge this after you satisfied the README.md

For the future, I afraid that current README.md is too big for the newbie, it should be put on another URL like python
https://github.com/microsoft/vscode-python#feature-details

[renkun-ken]

@Ikuyadeu Agreed. We could clean up the README later.

[renkun-ken]

LGTM. Merging now.