New feature: Export console width through accessor API

This change extends the linenoise API with a function returning the terminal width to the caller. It uses the internal getWindowSize() to determine the value.

Note: This pull request implicitly contains the pending pull requests/branches
  feature-cut-paste-buffer
  feature-hidden-input
  feature-page-updown

My use case was help texts printed to the terminal and the printing code auto-adapting the formatting to the terminal width.