Add display/setting of default values in component-create.

This adds some items of functionality relating to `component-create`'s default values:

### Prevents crash when not supplying username/repo for non-local components

Instead it just prompts user to fix, keeping all existing values (resolves #352).

### Sets default project repo/name to be `$USER` + `CWD`

```
component-create /tmp/test-component
repo (username/project): [timoxley/test-component]
description:
does this component have js? [yes]
does this component have css? [yes]
does this component have html? [yes]

      create : /tmp/test-component
      create : /tmp/test-component/index.js
      create : /tmp/test-component/template.html
      create : /tmp/test-component/test-component.css
      create : /tmp/test-component/Makefile
      create : /tmp/test-component/Readme.md
      create : /tmp/test-component/History.md
      create : /tmp/test-component/.gitignore
      create : /tmp/test-component/component.json
```

### or just `CWD` for local components:

```
component-create --local locals/test
name: [test]
description:
does this component have js? [yes]
does this component have css? [yes]
does this component have html? [yes]

      create : locals/test
      create : locals/test/index.js
      create : locals/test/template.html
      create : locals/test/test.css
      create : locals/test/component.json
```

### Also displays default values in `[` & `]`if they exist.

May want to consider something that allows setting default user to something other than `$USER`.