Add config.line_padding that specifies extra spacing above and below each line in the editor.
Merge pull request #14 from shalabhc/line_padding

Add config.line_padding - extra spacing above and below each line
Change input.register to take a definition table

Since it's desirable for inputs to have a description,
and this makes it match other register functions, such as
command.register().
Grammar

Closes #11.
Make chunks more effecient, implement buffer\usub()

buffer\usub() uses the raw scintilla buffer.
Rename buffer\usub() to buffer\sub() and change semantics

In order to match lua string.sub (handle negative offsets and
start > end), update buffer.md api doc.
Add note about issue #10
Better structure view for Python mode - show class and function defintions only.