Placehold: New feature added

* Feature: Placehold.it images.
* See #30 for more details.
* Tests added.

---

# Syntax:

#### Basic:

> place

will form:

```
<img src="http://placehold.it/30x30/" alt="" />
```

#### With Single Dimension:

> place-50

will form squared dimensions:

```
<img src="http://placehold.it/50x50/" alt="" />
```

#### With Complete Dimensions:

> place-50x40

will form:

```
<img src="http://placehold.it/50x40/" alt="" />
```

#### With Text:

> place-150x240-t=This%20is%20some%20text.

will form:

```
<img src="http://placehold.it/150x240/&text=This%20is%20some%20text" alt="" />
```
#### With Background Color:

> place-150x240-EEE-t=This%20is%20some%20text.

will form:

```
<img src="http://placehold.it/150x240/EEE/&text=This%20is%20some%20text" alt="" />
```

#### With Background and Foreground Colors:

> place-150x240-EEEDDD-888-t=This%20is%20some%20text.

will form:

```
<img src="http://placehold.it/150x240/EEEDDD/888/&text=This%20is%20some%20text" alt="" />
```

#### With Format:

> place-150x240-jpeg-EEEDDD-888222-t=This%20is%20some%20text.

will form:

```
<img src="http://placehold.it/150x240/jpeg/EEEDDD/888222/&text=This%20is%20some%20text" alt="" />
```