Add support for pending examples

[waterlink]

Closes #52

@miketheman take a look

[miketheman]

@waterlink very cool, tested today.

• works at the nested describe level
• works at the it level
• does not work at the Spec2.describe level - I tried Spec2.pending for all tests in the file
• does not work at top-level describe level after include Spec2::GlobalDSL

[waterlink]

@miketheman Great feedback! thank you. I'll merge this and will create an issue for edge cases that don't work. Will get to work on them soon.

[waterlink]

@miketheman I have just released current functionality as 0.10.0

➜  spec2.cr git:(master) release minor
[EXECUTOR] git commit -am "Bump 0.10.0"
[master a181a7d] Bump 0.10.0
 1 file changed, 4 insertions(+), 4 deletions(-)
[EXECUTOR] git tag v0.10.0
[EXECUTOR] git push origin master
Counting objects: 3, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 363 bytes | 0 bytes/s, done.
Total 3 (delta 1), reused 0 (delta 0)
remote: Resolving deltas: 100% (1/1), completed with 1 local objects.
To github.com:waterlink/spec2.cr
   01b002f..a181a7d  master -> master
[EXECUTOR] git push --tags
Total 0 (delta 0), reused 0 (delta 0)
To github.com:waterlink/spec2.cr
 * [new tag]         v0.10.0 -> v0.10.0
➜  spec2.cr git:(master)