Add support for Aliases on CASE expressions

[mostertb]

Hi

This PR allows CASE expressions to be aliased to fix issues #192 and #162

It specifically adds support for CASE ... END [AS] alias for both CASE formats with the AS keyword being optional.

Im a little out of my depth so any help or guidance tests or functionality that I have missed would be great.

I know that queries like CASE ... END expression [AS] alias such as SELECT CASE WHEN 2 IS NULL THEN 0 ELSE 2 END + 1 as 'foo' are not covered. Let me know if there is sufficient use-case for this scenario and I'll attempt it in a new PR

[codecov]

Codecov Report

Merging #203 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #203      +/-   ##
============================================
+ Coverage     99.75%   99.75%   +<.01%     
- Complexity     1751     1771      +20     
============================================
  Files            57       57              
  Lines          4126     4164      +38     
============================================
+ Hits           4116     4154      +38     
  Misses           10       10

[devenbansod]

LGTM 👍

[williamdes]

LGTM :)

[ibennetch]

Merged, thanks for your contribution!