Add husky to run markdownlint

[adam-resdiary]

I've added husky to run the markdownlint check as a pre-commit hook.

I've also added lint-staged, which does a couple of things for us:

• It will only run linters if any files have been changed matching certain patterns. For example, it'll only run the markdownlint check if any *.md files have been changed.
• It makes sure that your staged changes are valid so you don't fix a problem then forget to stage it before committing.

Fixes #741

Here's what it looks like if you try to commit a file with a failing markdown check:

$ git commit -m"Test commit"
husky > pre-commit (node v11.6.0)
Stashing changes... [started]
Stashing changes... [skipped]
→ No partially staged files found...
Running tasks... [started]
Running tasks for *.md [started]
npm run --silent markdownlint [started]
npm run --silent markdownlint [failed]
→
Running tasks for *.md [failed]
→
Running tasks... [failed]



× npm run --silent markdownlint found some errors. Please fix them and try committing again.

C:/Users/AdamConnelly/github.com/adamrpconnelly/promitor/CODE_OF_CONDUCT.md: 49: MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
CODE_OF_CONDUCT.md: 49: MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
husky > pre-commit hook failed (add --no-verify to bypass)

Here's what it looks like if there's no problems:

$ git commit -m"Test commit"
husky > pre-commit (node v11.6.0)
Stashing changes... [started]
Stashing changes... [skipped]
→ No partially staged files found...
Running tasks... [started]
Running tasks for *.md [started]
npm run --silent markdownlint [started]
npm run --silent markdownlint [completed]
Running tasks for *.md [completed]
Running tasks... [completed]
[husky 67c77e4] Test commit
 3 files changed, 1431 insertions(+)

Here's what it looks like if no files that need linting have been changed:

$ git commit -m"Test commit"
husky > pre-commit (node v11.6.0)
No staged files match any of provided globs.
[husky 610cc1f] Add husky to run markdownlint
 Date: Wed Oct 16 17:20:36 2019 +0100
 2 files changed, 1429 insertions(+)

@tomkerkhove if you want to try this out, just pull this branch, run npm install, make a change to a markdown file that causes an error and make a commit.

[tomkerkhove]

Awesome, thanks!