Fixes #2590

Fixes #2384

No need to covert these types.

Makes possible:

    BROWSER='"/Applications/Google Chrome.app/Contents/MacOS/Google Chrome" --extra --flag' hub browse

Fixes #2178

The GraphQL query has to accept the optional `endCursor` string variable
and output `pageInfo`:

    pageInfo {
      hasNextPage
      endCursor
    }

- default: `--color=auto`
- `--color` is equivalent to `--color=always`
- `--color=never` disables color for TTYs

Fixes https://github.com/github/hub/issues/2036

- print error output on stderr instead of stdout
- only prompt to open an issue if connected to terminal
- get rid of "always" option in the prompt
- empty response to the prompt is interpreted as a "no"

Design considerations:
- treats `--flag=VALUE` as equivalent of `--flag VALUE`
- treats `-abcVALUE` as equivalent to `-a -b -c VALUE`
- allows `--opt=false` as opposite of `--opt` or `--opt=true`
- retrieve values as string, slice of strings, bool, or int

Pick between white and black depending on which one is the first to
satisfy the contrast ratio of 7.0 (or 4.5 as fallback).

The `create` and `remote` commands needs to infer the current project
name from the project's directory. Instead of using the current working
directory, which doesn't necessarily match the project's toplevel
directory, use `git rev-parse --show-toplevel` which also respects the
`-C` global flag setting.

Fixes #1105, closes #1114

Instead of displaying plain text help, `hub help <command>` will now
search for man pages to display.

* If `man` is available, hub first tries to locate the local
  `../man/hub-<command>.1` file relative to the `hub` executable. If
  found, open it with `man`. If not, execute `man 1 hub-<command>`.

* Otherwise, resort to displaying plain text help.

Introduce a `console` package which encapsulates logic of printing out
to console. This provides flexibility of discarding output in tests.
It also allows us to unify format of printing to console (if we want to).

This fixes #801.

Removes duplicated test helper.