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
```