update install script

1. Create symbolic links instead of overwrite. This allow easy modify your local
   copy without reinstall.
2. Do not overwrite existing files on target directory and show warning.
3. Installation script renamed from `bootstrap` to `install`.
remove legacy powerline dist and use pure python implementation
update README

`bootstrap.sh` renamed into `install.sh`, so docs updated.
do not create symlink for `.gitmodules` on install
remove blank lines in `.gitignore`
ignore local config
add local configs

The local config ends with `.local` suffix, for example, `.gitconfig.local`,
`.bashrc.local`, `.bash/aliases.local`, etc.  They loaded after corresponding
global config to allow overwrite default values, settings and so on.
fix comment
fix system aliases for darwin (OSX)

The default `ls` distribution on darwin does not handle long `--` params. Use
alternate aliases.
add more git aliases
automatically colorize git output
update comments style and cleanup

1. Use `;` as comment string
2. Remove useless aliases
3. Add alias to show last commit
install git completion if needed
allow git completion work with aliases

For example, pressing `Tab` after `g ` show valid git commands, e.g. `add`,
`checkout`, etc. But now does not possible to pass some context to completion
function, so `gsp` does not show list stashes for pop, only only completion for
`git stash` command.
fix builtin completion for install script