issue show: add --comments flag to print notes & discussions for issue

[claytonrcarter]

See #207.

• default: do not print comments (current behavior)
• added -c/--comments flag to print comments as well as normal issue show output. I put this behind a flag b/c it became immediately obvious after I started using it locally that I have 2 use cases for viewing issues: viewing the issue metadata (descriptions, todos, labels, deadlines, etc) and viewing the discussions. If comments are printed on every show, it's tedious to scroll up past them to view the meta.
• does not print the system notes (like "soandso added label 'bug'" or '... changed the description')
• I had to upgrade the version of go-gitlab to 0.11.3 in order to get the Discussions API.

The format of the comments is not sacred. I just wanted to get something in place to get to discussion started.

Sample Output from https://gitlab.com/claytonrcarter/test/issues/2

$ ./lab issue show crc-testing 2 --comments

#2 Test issue for notes & discussions.
===================================
This is just an issues for testing notes and discussions in `lab`.
-----------------------------------
Project: claytonrcarter/test
Status: Open
Assignees:
Author: claytonrcarter
Milestone: None
Due Date: None
Time Stats: None
Labels:
WebURL: https://gitlab.com/claytonrcarter/test/issues/2

-----------------------------------
claytonrcarter commented at 2018-11-12 02:52:18.178 +0000 UTC

This comment is a note. (Comment 1.)

-----------------------------------
claytonrcarter started a discussion at 2018-11-12 02:52:39.251 +0000 UTC

    This comment is a discussion. (Comment 2.)

    -----------------------------------
    claytonrcarter commented at 2018-11-12 02:53:13.168 +0000 UTC

    This is a comment on the discussion. (Comment 2a.)

    -----------------------------------
    claytonrcarter commented at 2018-11-12 02:55:19.244 +0000 UTC

    This is another comment on the discussion. (Comment 2b.)

-----------------------------------
claytonrcarter started a discussion at 2018-11-12 02:55:28.528 +0000 UTC

    Another discussion. (Comment 3.)

-----------------------------------
claytonrcarter commented at 2018-11-12 02:56:23.967 +0000 UTC

Another comment. (Comment 4.)

[codecov]

Codecov Report

Merging #256 into master will decrease coverage by 0.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   69.58%   69.56%   -0.02%     
==========================================
  Files          44       44              
  Lines        2446     2481      +35     
==========================================
+ Hits         1702     1726      +24     
- Misses        575      583       +8     
- Partials      169      172       +3

Impacted Files	Coverage Δ
internal/gitlab/gitlab.go	42.78% <50%> (+0.15%)	⬆️
cmd/issue_show.go	92.4% <77.77%> (-7.6%)	⬇️
cmd/issue.go	63.15% <0%> (-5.27%)	⬇️

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[zaquestion]

Opened a support ticket with Codacy about the reviews taking FOREVER lately. I'm just gonna merge this in.