Adding the grep command

[RafBishopFox]

This PR adds a new command called grep. grep searches the contents of files for a given search pattern. The command is useful when you want to find files that have information that you care about without having to download the entire file. The search pattern can be a literal string or an RE2 regular expression. grep supports recursion and context (returning lines before and after the matching line).

For binary files, context results are disabled.

Some samples:
Basic search:

sliver (CRUEL_BEAVER) > grep -i root|polkit /etc/passwd

Found 2 matching lines for root|polkit in /etc/passwd

/etc/passwd: Line 1: root:x:0:0:root:/root:/bin/bash
/etc/passwd: Line 5: polkitd:x:483:483:User for polkitd:/var/lib/polkit:/usr/sbin/nologin

Same search with context:

sliver (CRUEL_BEAVER) > grep -i -C 2 root|polkit /etc/passwd

Found 2 matching lines for root|polkit in /etc/passwd

/etc/passwd (match on line 1)

root:x:0:0:root:/root:/bin/bash
systemd-timesync:x:497:497:systemd Time Synchronization:/:/usr/sbin/nologin
messagebus:x:484:484:User for D-Bus:/run/dbus:/usr/sbin/nologin


/etc/passwd (match on line 5)

messagebus:x:484:484:User for D-Bus:/run/dbus:/usr/sbin/nologin
mysql:x:60:60:MySQL database admin:/var/lib/mysql:/usr/sbin/nologin
polkitd:x:483:483:User for polkitd:/var/lib/polkit:/usr/sbin/nologin
scard:x:482:482:Smart Card Reader:/run/pcscd:/usr/sbin/nologin
wsdd:x:481:481:User for wsdd:/run/wsdd:/usr/sbin/nologin