Various edits to fine-tune behaviour on Linux

[lmmx]

• Add a check for xsel (an xclip alternative, but doesn't have xclip's -f filter flag, so can only write to one clipboard easily.
• Used this xclip -f flag to write to both primary and "clipboard" clipboards (Linux has 3).
• Read with scan() and write with writeChar(), because of trailing-newlines / multiple lines being mishandled.
• Move the stop() call advising to install "xclip (recommended) or xsel" into its own function, to avoid repetition.
• install -> install_github in README, and loading the library in the example