[tmux/en] Structural updates & additional content

+ Fixed some spelling, grammar, and punctuation issues
+ Separated terminal commands from keybinds
+ Rename learntmux.markdown.html to tmux.markdown.html
```
Line  48: Added additional items to the list of keybinds for power users
Line 104: Created section on configuring ~/.tmux.conf & added additional comments
Line 203: Added section including external resources
```
I am still contemplating this. I am not sure how to do this. How do you feel about:  
```
tmux          # Create an unnamed session
tmux new  # Equivalent to command above to create an unnamed session.
```
Also, while I was typing that, I see `unamed` is misspelled and should be `unnamed`