Add support for multiple `--message` options

This is for compatibility with git-commit(1)

It's not only the first line of text; it's the entire first "block" (or
paragraph) of text with newlines turned into spaces. This is for
compatibility with git-commit(1).

Fixes #1937

Per "TOKENS" section in ssh_config(5)

Fixes #1936

[ci skip]

[ci skip]

See #1404, closes #1929 [ci skip]

Add repo visibility and permission check for remote add command

`remote add` may now query the GitHub API to determine whether the
remote URL should be generated in writeable mode or not.

Add unix socket support

 Use 'scissors' to separate comments from PR messages 

This allows PR templates to include markdown headers with '#'

Add the command `hub issue show <NUMBER>`

This commit adds the subcommand hub issue view, which displays an issue as well as
lists its comments underneath.

Add ability to list requested reviewers with `%rs`

Fixes #1913

Fixes #1913

Adding unit tests for util methods

The default direction when sorting by `created` date (the default) is
descending. However, the default on the API backend changes to ascending
when the sort key is something different than `created`:
https://developer.github.com/v3/pulls/#list-pull-requests

This ensures that the default `direction: "desc"` is always passed to
API queries, regardless of the sort key, unless `--sort-ascending` was used.

Ref. https://github.com/github/hub/issues/1815#issuecomment-433222344

Update --labels documentation to show it will be created if not exists

by a factor of 3
Without this patch, using make -j
built every man page and html page 25 times

Now, man pages are always generated in a single ronn call
after all .ronn files are made.

Workarounds GNU make bug https://savannah.gnu.org/bugs/?54854

fix `compare` command if not on any branch

There is no need to detect the current branch if a range is provided
as an argument.

Fixes #1584.

Fixed CI status check exit code.