Update the plugins managment. Add the function 'omz-plugin' (inspired from bash-it)

plugins/available/mercurial/README.md

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+# Mercurial plugin
+### Usage
+Update .zshrc:
+
+1. Add name to the list of plugins, e.g. `plugins=(... mercurial ...)`
+   (that is pretty obvious).
+2. Switch to a theme which uses `hg_prompt_info`.
+
+   Or, customize the `$PROMPT` variable of your current theme to contain current folder mercurial repo info. This can be done by putting a custom version of the theme in `$ZSH_CUSTOM` or by changing `$PROMPT` in `.zshrc` after loading the theme.
+
+   The `robbyrussell` theme is used by default, so you need to modify `$PROMPT` var by adding `$(hg_prompt_info)` after `$(git_prompt_info)`, so it looks like this:
+
+   ```zsh
+   PROMPT='${ret_status}%{$fg_bold[green]%}%p %{$fg[cyan]%}%c %{$fg_bold[blue]%}$(git_prompt_info)$(hg_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}'
+   ```
+
+3. Initialize additional vars used in plugin. So in short put next in **.zshrc**:
+
+   ```
+   ZSH_THEME_HG_PROMPT_PREFIX="%{$fg_bold[magenta]%}hg:(%{$fg[red]%}"
+   ZSH_THEME_HG_PROMPT_SUFFIX="%{$reset_color%}"
+   ZSH_THEME_HG_PROMPT_DIRTY="%{$fg[magenta]%}) %{$fg[yellow]%}✗%{$reset_color%}"
+   ZSH_THEME_HG_PROMPT_CLEAN="%{$fg[magenta]%})"
+   ```
+
+### What's inside?
+#### Adds handy aliases:
+###### general
+* `hgc` - `hg commit`
+* `hgb` - `hg branch`
+* `hgba` - `hg branches`
+* `hgbk` - `hg bookmarks`
+* `hgco` - `hg checkout`
+* `hgd`  - `hg diff`
+* `hged` - `hg diffmerge`
+
+###### pull and update
+* `hgi` - `hg incoming`
+* `hgl` - `hg pull -u`
+* `hglr` - `hg pull --rebase`
+* `hgo` - `hg outgoing`
+* `hgp` - `hg push`
+* `hgs` - `hg status`
+* `hgsl` - `hg log --limit 20 --template "{node|short} | {date|isodatesec} | {author|user}: {desc|strip|firstline}\n"`
+
+###### this is the 'git commit --amend' equivalent
+* `hgca` - `hg qimport -r tip ; hg qrefresh -e ; hg qfinish tip`
+
+###### list unresolved files (since hg does not list unmerged files in the status command)
+* `hgun` - `hg resolve --list`
+
+#### Displays repo branch and directory status in prompt
+This is the same as git plugin does.
+
+**Note**: Additional changes to **.zshrc**, or using a theme designed to use `hg_prompt_info`, are required in order for this to work.
+
+### Mantainers
+[ptrv](https://github.com/ptrv) - original creator
+
+[oshybystyi](https://github.com/oshybystyi) - created this README and know how most of code works