Add zsh-mvn-contexts plugin

[unixorn]

Description

Add mvn-contexts now that artemy/zsh-mvn-contexts#1 is closed.

Type of changes

• A link to an external resource like a blog post
• Add/remove a framework
• Add/remove a plugin
• Add/remove a tab completion
• Add/remove a theme
• Text cleanups/typo fixes

Copyright Assignment

• This document is covered by the BSD License, and I agree to contribute this PR under the terms of the license.

Checklist:

• I have confirmed that the link(s) in my PR are valid.
• My entries are single lines and are in the appropriate (plugins, themes or completions) section, and in alphabetical order in their section.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.
• Any added completions have a license file in their repository.
• Any added plugins have a license file in their repository.
• Any added themes have a screen shot and a license file in their repository.
• I have stripped leading and trailing zsh- or oh-my-zsh- substrings from the link name. This makes it easier to find plugins/themes/completions by name since there aren't big clusters in the O and Z sections of the list.

[codeclimate]

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