Fix: Support autofix at the very start of blocks

[lydell]

Previously, if a rule added an autofix at index 0 of a code block, all
code from the start of the markdown file until the start of that code
block would be removed. Now, the autofix is applied as expected.

Fixes lydell/eslint-plugin-simple-import-sort#22

[lydell]

The build seems to have failed on Node.js 9 for reasons not related to this PR:

$ node --version
v9.11.2
$ npm --version
5.6.0
$ nvm --version
0.34.0
install.npm
2.26s$ npm install 
0.62s$ npm test
> eslint-plugin-markdown@1.0.0 test /home/travis/build/eslint/eslint-plugin-markdown
> npm run lint && npm run test-cov
> eslint-plugin-markdown@1.0.0 lint /home/travis/build/eslint/eslint-plugin-markdown
> eslint Makefile.js lib/**/*.js tests/lib/plugin.js
internal/modules/cjs/loader.js:550
    throw err;
    ^
Error: Cannot find module 'readable-stream'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:548:15)
    at Function.Module._load (internal/modules/cjs/loader.js:475:25)
    at Module.require (internal/modules/cjs/loader.js:598:17)
    at require (internal/modules/cjs/helpers.js:11:18)
    at Object.<anonymous> (/home/travis/build/eslint/eslint-plugin-markdown/node_modules/concat-stream/index.js:1:78)
    at Module._compile (internal/modules/cjs/loader.js:654:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:665:10)
    at Module.load (internal/modules/cjs/loader.js:566:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:506:12)
    at Function.Module._load (internal/modules/cjs/loader.js:498:3)

[lydell]

Anyone wants to review this bugfix?

[btmills]

LGTM, thank you for fixing this @lydell! I just verified that this will also fix #117. I restarted the Node.js v9 build on Travis. Once that passes, I'll get this merged.

[lydell]

Thanks!