In the form of grep where the 2nd argument is an Array, if one of the items in the Array points to a directory, ShellJS will crash

[bedney]

Node version (or tell us if you're using electron or some other framework):

10.17.0

ShellJS version (the most recent version/Github branch you see the bug on):

0.8.3

Operating system:

Mac OSX

Description of the bug:

When using grep(), if you're using the form where you hand in a file_array and one of the files is actually a directory, ShellJS will crash.

Example ShellJS command to reproduce the error:

This will crash when it reaches directoryname:

sh = require('shelljs');
let my_file_array = ['filename1', 'filename2', 'directoryname'];
sh.grep(/hi there/, my_file_array);

[nfischer]

The trace I got was:

[PATH REDACTED]/shelljs/src/common.js:399
        throw e;
        ^

Error [ShellJSInternalError]: EISDIR: illegal operation on a directory, read
    at Object.readSync (fs.js:524:3)
    at tryReadSync (fs.js:349:20)
    at Object.readFileSync (fs.js:386:19)
    at [PATH REDACTED]/shelljs/src/grep.js:55:45
    at Array.forEach (<anonymous>)
    at Object._grep ([PATH REDACTED]/shelljs/src/grep.js:49:9)
    at Object.grep ([PATH REDACTED]/shelljs/src/common.js:384:25)
    at Object.<anonymous> (/home/nfischer/shelljscrash/index.js:4:4)
    at Module._compile (internal/modules/cjs/loader.js:1133:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1153:10) {
  errno: -21,
  syscall: 'read',
  code: 'EISDIR'
}

---

I think this is WAI. grep does not support directory arguments (see https://github.com/shelljs/shelljs#grepoptions-regex_filter-file--file-). We don't currently support the -R flag, although I filed #998 to track this.