Add filter by formula functionality (#70)

* add filter by formula functionality

* add linebreak

* fix styleci error

* add example to readme

Author: chriship

README.md

@@ -94,6 +94,13 @@ Airtable::where('id', '5')->get();
 Airtable::where('id', '>', '5')->get();
 ```
 
+### Filter records by formula
+- When using `where` is not enough you may need to pass in raw filter values.
+- [Airtable reference](https://support.airtable.com/docs/formula-field-reference)
+``` php
+Airtable::table('tasks')->filterByFormula('OR({id} = "abc", {id} = "def", {id} = "ghi")')->get();
+```
+
 #### Sorting records
 
 - First argument is the column name

src/Airtable.php

@@ -106,6 +106,13 @@ public function where($column, $operator, $value = null)
         return $this;
     }
 
+    public function filterByFormula($formula)
+    {
+        $this->api->addFilterByFormula($formula);
+
+        return $this;
+    }
+
     public function orderBy(string $column, string $direction = 'asc')
     {
         $this->api->addSort($column, $direction);

src/Api/AirtableApiClient.php

@@ -48,6 +48,13 @@ public function addFilter($column, $operation, $value): AirtableApiClient
         return $this;
     }
 
+    public function addFilterByFormula(string $formula): AirtableApiClient
+    {
+        $this->filters[] = $formula;
+
+        return $this;
+    }
+
     public function addSort(string $column, string $direction = 'asc'): AirtableApiClient
     {
         if ($direction === 'desc') {