feat(add): Add --exact option to lerna add

[mcous]

Description

This PR adds an --exact flag to lerna add for installing dependencies at a fixed version, and it ensures that config.commands.add.exact is able to override the setting.

Motivation and Context

Both npm install and yarn add are able to install dependencies at a fixed version using --save-exact and --exact, respectively. lerna has an exact option available in init and publish, and some users expect that behavior to also exist in add.

Closes #1470

How Has This Been Tested?

I updated the unit tests to cover the new functionality. I also yarn linked lerna and tested on a local monorepo:

• lerna add --exact to add new remote dependency
• lerna add --exact to move ^ remote dependency to exact
• lerna add with commands.add.exact in lerna.json set to true
• lerna add with exact in lerna.json set to true
• lerna add (with config in lerna.json) to add new local dependency
• lerna add (with config in lerna.json) to move ^ local dependency to exact

Setup

• macOS 10.13
• Node v8.11.2
• npm 6.1.0
• yarn 1.7.0

During the course of updating the unit tests, I noticed that the pkg-matchers extend matchers were not actually returning the correct objects, so I fixed those up as well. The object returned should be...

{
  pass: boolean,
  message: () => string
}

...and message for pass: true should be for the .not case. See jest docs.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[mcous]

@evocateur I noticed some activity on master over the last few days, so I wanted to ping you to see if a) you had any interest in merging this PR and b) if there's anything I can do in terms of fixing, rebasing, squashing, etc. to get this PR mergable for you.

From lurking in the issues you seem pretty busy, so no rush! I'm a fan (and user) of the project and was just hoping to help out 😄

[evocateur]

@mcous Sorry for the delay, been really busy at work. I did some reorganizing that caused conflicts, so I'm gonna resolve those and merge this today. Thanks again for your patience!

[evocateur]

CI failures are unrelated, no worries.

[lock]

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