Add `use_non_breaking_spaces` theme option, use it in ascii theme
Replace .replace(' ', NBSP) call with self.escape call

Dividers need to be escaped as well.

Fixes #1027
Fall back to ASCII-only theme in case of non-unicode locale
Separate some issues into “Vim issues” section in troubleshooting
Also add “Shell issues” section
Move LANG setting to run function, remove useless BINDFILE variable
Add another answer to troubleshooting section
Use &encoding as powerline encoding in Vim

Among other benefits (i.e. correct fall back to ascii theme when Vim uses
non-unicode encoding) this should also fix travis tests: apparently travis is
not setting LANG or similar environment variables to some unicode locale, so
tests are failing.