Add inflector option

For now you can specify inflector for header column:

Example:

```ruby
csv.headers do |csv|
  csv.columns :name, :age, :location
  csv.mappings name: 'Full name', location: 'Region'
  csv.inflector :titleize
end
```

or

```ruby
CsvShaper.configure do |config|
  config.header_inflector = :titleize
end
```
or

```ruby
csv_string = CsvShaper::Shaper.encode(header_inflector: :titleize) do |csv|
  ...
end
```
I can't help but wondering if supporting something that responds to `call` would be a nice little touch here.  It would give much more flexibility.

```ruby
@mappings[column] || (@inflector.respond_to?(:call) ? @inflector.call(column) : column.to_s.send(@inflector))
```

Although that code looks like it could be cleaned up.. :)    It's not really needed for us right now but might be nice to improve in the future.