Improve searching for correct cop candidates
Improve searching for correct cop candidates

Follow up of https://github.com/bbatsov/rubocop/pull/5300#issuecomment-353717122.

This commit changed the search approach of the correct cop candidates
from prefix-match by regular expression to `StringUtil.similarity`.

## Before

When there is no prefix-match, candidates are not displayed.

```console
% rubocop --only Lint/RedundanWithI
Inspecting 1083 files

0 files inspected, no offenses detected
Unrecognized cop or department: Lint/RedundanWithI.
```

## After

When there is no prefix-match, candidate is displayed.

```console
% rubocop --only Lint/RedundanWithI
Inspecting 1083 files

0 files inspected, no offenses detected
Unrecognized cop or department: Lint/RedundanWithI.
Did you mean? Lint/RedundantWithIndex
```

Cop candidates different from user's expectations may be displayed,
however I think that there is no problem in cop names of similarity to
typing miss.

```console
% rubocop --only Lint/RedundanWithO
Inspecting 1083 files

0 files inspected, no offenses detected
Unrecognized cop or department: Lint/RedundanWithO.
Did you mean? Lint/RandOne, Lint/RedundantWithObject
```

## Other Information

Initially I thought about using the module of did_you_mean which is a
standard library of Ruby 2.3 or higher. However, since the following
code does not work unless it is Ruby 2.4 or higher, I decided to use
`StringUtil.similarity`.

```ruby
DidYouMean::SpellChecker.new(dictionary: cop_names).correct(name)
```

Also, since it displays multiple cop candidates, this commit did not
use `NameSimilarity#find_similar_name`.

deivid-rodriguez/rubocop,palkan/rubocop,tdeo/rubocop,rrosenblum/rubocop,maxjacobson/rubocop,jmks/rubocop,panthomakos/rubocop,petehamilton/rubocop,bbatsov/rubocop,bquorning/rubocop,tejasbubane/rubocop,palkan/rubocop,haziqhafizuddin/rubocop,meganemura/rubocop,deivid-rodriguez/rubocop,sue445/rubocop,palkan/rubocop,rrosenblum/rubocop,jfelchner/rubocop,petehamilton/rubocop,urbanautomaton/rubocop,meganemura/rubocop,sue445/rubocop,jmks/rubocop,scottmatthewman/rubocop,jfelchner/rubocop,maxjacobson/rubocop,tdeo/rubocop,vergenzt/rubocop,vergenzt/rubocop,smakagon/rubocop,petehamilton/rubocop,jmks/rubocop,tejasbubane/rubocop,pocke/rubocop,tejasbubane/rubocop,maxjacobson/rubocop,mikegee/rubocop,tdeo/rubocop,akihiro17/rubocop,pocke/rubocop,akihiro17/rubocop,haziqhafizuddin/rubocop,akihiro17/rubocop,smakagon/rubocop,bquorning/rubocop,jfelchner/rubocop,rrosenblum/rubocop,mikegee/rubocop,panthomakos/rubocop,mikegee/rubocop,bbatsov/rubocop,scottmatthewman/rubocop,bquorning/rubocop,panthomakos/rubocop,meganemura/rubocop,urbanautomaton/rubocop,sue445/rubocop,deivid-rodriguez/rubocop,vergenzt/rubocop