Add text filter and mechanism for registering builtin filters

docs/dispatcher/filters/text.md

@@ -0,0 +1,47 @@
+# Text
+
+This filter can be used for filter text [Message](../../api/types/message.md), 
+any [CallbackQuery](../../api/types/callback_query.md) with `data`, 
+[InlineQuery](../../api/types/inline_query.md) or
+[Poll](../../api/types/poll.md) question.
+
+Can be imported:
+
+- `#!python3 from aiogram.dispatcher.filters.text import Text`
+- `#!python3 from aiogram.dispatcher.filters import Text`
+- `#!python3 from aiogram.filters import Text`
+
+## Specification
+
+| Argument | Type | Description |
+| --- | --- | --- |
+| `text` | `#!python3 Optional[Union[str, List[str], Set[str], Tuple[str]]]` | Text equals value or one of values |
+| `text_contains` | `#!python3 Optional[Union[str, List[str], Set[str], Tuple[str]]]` | Text contains value or one of values |
+| `text_startswith` | `#!python3 Optional[Union[str, List[str], Set[str], Tuple[str]]]` | Text starts with value or one of values |
+| `text_endswith` | `#!python3 Optional[Union[str, List[str], Set[str], Tuple[str]]]` | Text ends with value or one of values |
+| `text_ignore_case` | `#!python3 bool` | Ignore case when checks (Default: `#!python3 False`) |
+
+!!! warning
+    
+    Only one of `text`, `text_contains`, `text_startswith` or `text_endswith` argument can be used at once.
+    Any of that arguments can be string, list, set or tuple of strings.  
+
+## Usage
+
+1. Text equals with the specified value: `#!python3 Text(text="text")  # value == 'text'`
+1. Text starts with the specified value: `#!python3 Text(text_startswith="text")  # value.startswith('text')`
+1. Text ends with the specified value: `#!python3 Text(text_endswith="text")  # value.endswith('text')`
+1. Text contains the specified value: `#!python3 Text(text_endswith="text")  # value in 'text'`
+1. Any of previous listed filters can be list, set or tuple of strings that's mean any of listed value should be equals/startswith/endswith/contains: `#!python3 Text(text=["text", "spam"])`
+1. Ignore case can be combined with any previous listed filter: `#!python3 Text(text="Text", text_ignore_case=True)  # value.lower() == 'text'.lower()`
+
+## Allowed handlers
+
+Allowed update types for this filter:
+
+- `message`
+- `edited_message`
+- `channel_post`
+- `edited_channel_post`
+- `inline_query`
+- `callback_query`

docs/dispatcher/observer.md

@@ -39,7 +39,7 @@ In this handler can be bounded filters which can be used as keyword arguments in
 
 ### Registering bound filters
 
-Bound filter should be subclass of [BaseFilter](filters/base_filter.md)
+Bound filter should be subclass of [BaseFilter](filters/base.md)
 
 `#!python3 <observer>.bind_filter(MyFilter)`