Create Transformation & Feature Eng Pipelines

Using something like JSON to write code pipelines (as opposed to serializing functions -- since I don't think there's a good way to serialize Julia code in a way that will keep it consistent).

Doesn't necessarily have to be tied to Julia (could try implementing it in other languages, like Python). The pipeline could be planned out in python but carried out in Julia, for example.

## Example

Say you wanted to:
* Log scale column `"number-of-views"`
* Convert column `"timestamp"` to `DateTime` format
* Extract the day of the week from the column `"timestamp"`

The pipeline file might look like this:
```json
[
{
"operation":"log-scale",
"columns": [ "number-of-views" ],
"parameters": { "base": 10 }
},
{
"operation": "parse-datetime",
"columns": [ "timestamp" ],
"parameters":{
"inplace": true,
"format": "yyyy-mm-dd HH:MM:SS"
}
},
{
"operation": "extract-dow",
"columns": [ "timestamp" ],
"parameters": {
"new-column-name": "day-of-week"
}
}
]
```