Improve snap packaging

Signed-off-by: N林博仁(Buo-Ren Lin) <Buo.Ren.Lin@gmail.com>

Currently shellcheck is only provided by the edge channel, should remove the --channel argument after it is in candidate/stable.
Signed-off-by: N林博仁(Buo-Ren Lin) <Buo.Ren.Lin@gmail.com>

This patches uses the following gitignore syntax so that only entries in the root folder is ignored, it is suggested to apply it to existing rules as well.

```
A leading slash matches the beginning of the pathname. For
example, "/*.c" matches "cat-file.c" but not
"mozilla-sha1/sha1.c".
```

Refer-to: gitignore(5) manpage
Signed-off-by: N林博仁(Buo-Ren Lin) <Buo.Ren.Lin@gmail.com>

Otherwise it will be blocked by Apparmor with the following message:

```
$ shellcheck script
audit: type=1400 audit(TIMESTAMP): apparmor="DENIED" operation="open" profile="snap.shellcheck.shellcheck" name=2F6D656469612F4C696E2D42756F2D52656E2F57696E646F7773205553422F717569636B72756E pid=10175 comm="shellcheck" requested_mask="r" denied_mask="r" fsuid=FSUID ouid=OUID
script: script: openBinaryFile: permission denied (Permission denied)
```

NOTE:

* This plug is not Auto-connect plug, it has to be manually connected by user with `snap connect shellcheck:removable-media :removable-media`
* Currently files under /mnt is not checkable as snapd doesn't provide an interface for it for now.

Refer-to: Interfaces reference - Snaps are universal Linux packages <https://docs.snapcraft.io/reference/interfaces>
Signed-off-by: N林博仁(Buo-Ren Lin) <Buo.Ren.Lin@gmail.com>

Consider type a valid command in sh

[type](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/type.html) *is* defined by POSIX, and even the bourne shell has the `type` command.

Simplify Dockerfile

README - Travis script for updating shellcheck ver

Shellcheck in default Travis builds is currently a version behind

Use multi stage Dockerfile to greatly simplify build.

Merge branch 'simplify_dockerbuild' of https://github.com/pratikmallya/shellcheck into pratikmallya-simplify_dockerbuild

Move library into src/

Use multi stage Dockerfile to greatly simplify build.

custom-setup:

- http://cabal.readthedocs.io/en/latest/developing-packages.html#custom-setup-scripts
- https://www.well-typed.com/blog/2015/07/cabal-setup-deps/

Bounds:
- containers-0.5 is required if Data.Map.Strict
- parsec-3.0 for Text.Parsec
- json-0.3.6 for makeObj

Check for calls to `which`.

MacOS support for nextnumber