Improve argument validation for maktaba#buffer#Overwrite().

- Check that 1 <= N <= line('$') for both startline and endline.
- Check that startline <= endline when using the Python implementation.
- Add tests for the above.
Merge pull request #83 from malcolmr/verify_buffer_overwrite_args

Improve argument validation for maktaba#buffer#Overwrite().
Return '.' instead of '' from path#MakeRelative
Regen docs with latest vimdoc
Merge pull request #85 from google/makerelative

Return '.' instead of '' from path#MakeRelative
Regen docs with vimdoc-0.4.1
Don't return value from Plugin.Flag({flag}, {value}) and clean up @usage
Clarify some "equivalence" examples in Plugin.Flag() docs
Merge pull request #87 from google/flag_return

Don't return value from Plugin.Flag({flag}, {value}) and clean up @usage
Regen docs with vimdoc-0.5.0 (changes in helloworld)
Don't drop empty words from syscall commands (#88)
Simplify SoftShellEscape regex (*->+, use [:alnum:])
Merge pull request #89 from google/syscall_empty

Don't drop empty words from syscall commands
Add .travis.yml file
.travis.yml: Set up vim-gnome and xvfb display
Fix rtp.vroom failing when repo dir isn't "maktaba"
Merge pull request #90 from google/vroom

Fix rtp.vroom failing when repo dir isn't "maktaba"
.travis.yml: Set language to "generic"
Use latest vroom and enable neovim mode tests in Travis CI
Check g:weirdplugin.name in vroom, for clarity and nicer failures