Allows variables to be set using the plessc command.

lessphp supports [setting variables from PHP][1], but the `plessc`
command doesn't provide a way to do this.  This patch adds support
for defining variables with the "-d" flag, e.g.

```bash
./plessc -d=foo:bar -d=blarg:w00t style.less
```

can be used to define the @foo and @blarg variables with "bar" and
"w00t" respectively.

The same caveats as per usual apply to [setting less strings][1].  Note
that, because colons are used for splitting the variable name and value,
any literal colons should be escaped with a backslash, e.g.

```bash
./plessc -d=foo:bar\\:blarg style.less
./plessc -d='foo:bar\:blarg' style.less
```

[1]: http://leafo.net/lessphp/docs/#setting_variables_from_php