Use word under cursor for previewDataframe, nrow #137

[andycraig]

Closes #137

What problem did you solve?

This PR makes two changes:

1. If there is no selection, the word under the cursor is used for r.previewDataframe and the r.nrow family of commands (r.nrow, r.length, r.head, r.thead, r.names);
2. If multiple lines are selected, the r.nrow family of commands apply the function(s) to the whole selection, instead of to each line in the selection.

Before

Commands don't work when cursor is on iris:

Functions applied to each line in selection:

After

@MaTo04 Thank you for adding the r.nrow family of commands! Do you think these changes are okay?

[andycraig]

I shortened the title to be within the recommended 50 characters for a commit subject.

[Ikuyadeu]

@andycraig Great! Thank you for your fix!
After changing the LICENSE, I will publish this version.

[andycraig]

@Ikuyadeu Thank you!