Use posix paths with patterns passed to globby.

globby uses the fast-glob package, which states this in its README (RE:
patterns):

> ⚠️ Always use forward-slashes in glob expressions (patterns and ignore
> option). Use backslashes for escaping characters.

Previously, we were using `path.join` which will use the OS specific
separator and therefore fail to match files when invoked with a
directory path directly.

---

This also corrects for a bug in globby (to be reported/fixed separately)
where the `cwd` option is defaulted to `process.cwd()` _without_ posix
correction, and then subsequently an assertion is made that a given path
"starts with" the cwd.

package.json

@@ -34,6 +34,7 @@
     "commander": "^3.0.0",
     "globby": "^10.0.1",
     "ora": "^4.0.2",
+    "slash": "^3.0.0",
     "tmp": "^0.1.0",
     "workerpool": "^5.0.1"
   },

src/runner.js

@@ -1,8 +1,10 @@
 const http = require('http');
 const https = require('https');
 const { writeFileSync } = require('fs');
-const { resolve, extname, join } = require('path');
+const { join } = require('path').posix;
+const { resolve, extname } = require('path');
 const colors = require('colors/safe');
+const slash = require('slash');
 const globby = require('globby');
 const ora = require('ora');
 const queue = require('async-promise-queue');
@@ -182,7 +184,14 @@ async function getAllFiles(paths) {
     return path;
   });
 
-  const files = await globby(patterns, { absolute: true, gitignore: true });
+  const files = await globby(patterns, {
+    // must specify a properly escaped `cwd` because globby infers from
+    // process.cwd() directly and without correcting back to posix paths
+    // asserts if the individual file path isn't "in" the cwd
+    cwd: slash(process.cwd()),
+    absolute: true,
+    gitignore: true,
+  });
   if (files.length < 1) {
     throw new NoFilesError();
   }