0.4.0

- Added support for locally-managed plugins
    - Plug expression should start with either `/`, `~`, or `$`
- Added `frozen` option for not updating/installing unless explicitly specifed
- Errors during help tag generation will be ignored
- Automatic retrial on timeout
- Added `R` keybinding for retrying failed update/installation
- Workaround for screen freeze problem on GUI MacVim
