Issue #172

Previously, `cheat` would exit if run by `root`. The rationale was:

If `cheat` was run by an unprivileged user (`chris`, for example), the
`$DEFAULT_CHEAT_DIR` would default to `/home/chris/.cheat`. If cheat was
run via `sudo`, however, the `$DEFAULT_CHEAT_DIR` would suddenly be
`/root/.cheat`.

Presuming that those individual user cheat dirs actually contained cheat
sheets, this could cause confusion, because cheat sheets accessible to
one user (`chris`) would not be accessible to the other (`root`). Thus,
cheatsheets would appear and disappear, depending on which user was
running `cheat`. (This would only be an issue, of course, for cheat
sheets that existed within the respective users' cheat dirs. System-wide
cheat sheets would be unaffected.)

`cheat` was thus programmed to gracefully fail when run as `root` to
prevent that possible confusion.

However, I'm backing away from that reasoning, because:

1. It's causing a headache for real users who'd like to run `cheat` as
root

2. Other venerable programs (`vim`, etc.) suffer from the same problem,
but nobody seems to mind enough to do anything about it. Thus, I suppose
the laissez-faire approach is essentially the sanctioned "solution" to
this problem.

3. Users sufficently troubled by this confusion may rememdy the problem
manually by setting environment variables (`$DEFAULT_CHEAT_DIR`, etc.)

Thus, `cheat` no longer complains if run by `root`.

Version-bumped to 2.1.0.
Merge pull request #178 from chrisallenlane/root

Issue #172
Version bump.
Issue #175

Noted `tldr` as a closely-related project in the `README`.
Merge pull request #179 from chrisallenlane/dev

Issue #175