Multiline configuration values

This PR adds support for multiline string values in the config.  The only, but pretty important, place string config values are used are in `config.startCode` and `config.endCode` strings.

### API Changes

  * `ConfigSettings::startCode` and `ConfigSettings::endCode` are now `std::string`
  * Code paths that expected `startCode` and `endCode` to be `char*` updated to call `.c_str()` 
  * `ConfigSettings::readSettings` can now read `"""` delimited multiline strings

### Trivial changes
  
  * Trailing whitespace removal in `fffProcessor.h`

### New config format additions

Multiline strings begin with `"""` and end with a `"""` appearing on a line by itself.  Everything in between is read line by line and concatenated with newlines into the resulting value.

#### Example config

```sh
## A default config for makerbot gcode
gcodeFlavor = 2 # Makerbot

# Layers
layerThickness = 270
fanFullOnLayerNr = 4

# Speeds
moveSpeed = 140

# Start and End gcode
startCode = """
; This is coming from the config
"""

endCode = """
;     such end code
;

; very wow
"""
```