Add an action to "share" a result.

- Refactor some functions.
- Update some test.
- Add a function to browse Dash' docsets.
Update doc strings.
Rename helm-dash-copy-to-clipboard.

- `helm-dash-add-to-kill-ring' instead of copy-to-clipboard.
- Add a test for this new function.
Update README.

- Add info about customize `helm-dash-browser-url'.
Merge pull request #49 from areina/copy-to-clipboard

WIP: Add an action to "share" a result.