Re-add EditorConfig support (undo #3213) (#3255)

* Revert "Revert "Respect EditorConfig settings" (#3213)"

This reverts commit d2241fc0d5.

* Comment out EditorConfig docs

See https://github.com/prettier/prettier/pull/3213#issuecomment-343009769

* editorconfig: Support `indent_size = 0`

See https://github.com/prettier/prettier/pull/2760#discussion_r137447715
and https://github.com/josephfrazier/editorconfig-to-prettier/commit/c38b84c42a2e022067c104c494298a8c9533e7a7

* Revert "Comment out EditorConfig docs"

This reverts commit ddfa529c55cac4853a1e76e00c8b5e3ef158c01f.

* Mark EditorConfig functionality as v1.9.0+

See https://github.com/prettier/prettier/pull/3255#discussion_r150432508

* editorconfig: Upgrade editorconfig-to-prettier to 0.0.4

* editorconfig: Only enable for CLI, by default

https://github.com/prettier/prettier/pull/3255#issuecomment-348420546

* editorconfig: Add tests confirming that editorconfig is ignored by default in the API

https://github.com/prettier/prettier/pull/3255#issuecomment-348420546

* editorconfig: Add/fix CLI option parsing

* editorconfig: Move docs from configuration.md to options.md

* editorconfig: Add `oppositeDescription` to show docs for `--no-editorconfig`

Addresses https://github.com/prettier/prettier/pull/3255#discussion_r154542792

* editorconfig: Update test snapshots

* editorconfig: Remove unnecessary options parsing code

Addresses https://github.com/prettier/prettier/pull/3255#discussion_r154544560

* editorconfig: Move docs from options.md to api.md and cli.md

Addresses https://github.com/prettier/prettier/pull/3255#discussion_r154545979

* resolveConfig: return null if both .prettierrc and .editorconfig are missing

Addresses https://github.com/prettier/prettier/pull/3255#discussion_r154574613

* Don't add now-failing tests

The way these tests work, both `tests_integration/cli/config/.prettierrc`
and `.prettierrc` apply to `tests_integration/cli/config/editorconfig/file.shouldnotexist`,
so the test wouldn't work even on master. Here's a way to confirm that:

```js
const path = require('path')
const assert = require('assert')
const prettier = require('./')

const file = './tests_integration/cli/config/editorconfig/file.shouldnotexist'
console.log(prettier.resolveConfig.sync(file))
assert(prettier.resolveConfig.sync(file) === null)
```

docs/api.md

@@ -49,6 +49,12 @@ prettier.resolveConfig(filePath).then(options => {
 });
 ```
 
+If `options.editorconfig` is `true` and an [`.editorconfig` file](http://editorconfig.org/) is in your project, Prettier will parse it and convert its properties to the corresponding prettier configuration. This configuration will be overridden by `.prettierrc`, etc. Currently, the following EditorConfig properties are supported:
+
+* `indent_style`
+* `indent_size`/`tab_width`
+* `max_line_length`
+
 Use `prettier.resolveConfig.sync(filePath [, options])` if you'd like to use sync version.
 
 ## `prettier.clearConfigCache()`