Support multi-root workspace (#22)

* Support multi-root workspace

1. Upgrade to vscode 1.1.10 to get the `getWorkspaceFolder` api
2. On change of tab refresh the files cache if workspace folder changes
3. Show info message when using plugin when workspacePath is null

* Fix code review comment and update documentation

* Update documentation

Author: rrag

README.md

@@ -10,7 +10,16 @@ Alternatively, you can press open command palette `F1` and search for `Relative
 First, you will need to install Visual Studio Code. In the command palette (`Ctrl-Shift-P` or `Cmd-Shift-P`) select `Install Extension` and choose `RelativePath`.
 
 ## Important
-Your workspace may be really big, so please wait for the initial file list to be created. This will happen only once.
+
+### In Multi root workspaces:
+
+Everytime you switch to a file from a different folder the files in that folder are indexed and
+cached to improve search performance. If you have multiple large folders part of a workspace
+frequent switches between folders might slow you down.
+
+### In Single project workspace:
+The caching of the filelist in the project happens only once. If your workspace contains a lot of files
+please wait for the initial file list to be created.
 
 ## Options
 The following Visual Studio Code settings are available for the RelativePath extension. They can be set in user preferences (`ctrl+,` or `cmd+,`) or workspace settings (.vscode/settings.json).