Config line breaks

[jake-stewart]

This pull requests adds the line break syntax proposed in #6600.

With these changes, lines beginning with a backslash will be joined with the line above. The leading whitespace and backslash are removed.

map ctrl+1 combine
    \ : send_text all \
    \ : set_colors ~/path/to/theme.conf

becomes

map ctrl+1 combine : send_text all \ : set_colors ~/path/to/theme.conf

No whitespace is added upon line joining which allows for word breaks.

map ctrl+1 send_
    \text all \

becomes

map ctrl+1 send_text all \

The changes affect both the python and go config parsers.
These changes pass all tests as well as some additional ones.

[kovidgoyal]

Please fix the failing test which is about pyhton code formatting and also add a line or two in docs/conf.rst documenting this syntax.

[jake-stewart]

sorted @kovidgoyal