Keyword/filtering API

This adds the 5 methods for the mastodon API that deal with keyword
filtering:

GET /api/v1/filters
POST /api/v1/filters
GET /api/v1/filters/:id
PUT /api/v1/filters/:id
DELETE /api/v1/filters/:id

Closes #71

src/entities/filter.rs

@@ -0,0 +1,27 @@
+/// Represents a single Filter
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct Filter {
+    id: String,
+    phrase: String,
+    context: Vec<FilterContext>,
+    expires_at: Option<String>, // TODO: timestamp
+    irreversible: bool,
+    whole_word: bool,
+}
+
+/// Represents the various types of Filter contexts
+#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
+pub enum FilterContext {
+    /// Represents the "home" context
+    #[serde(rename = "home")]
+    Home,
+    /// Represents the "notifications" context
+    #[serde(rename = "notifications")]
+    Notifications,
+    /// Represents the "public" context
+    #[serde(rename = "public")]
+    Public,
+    /// Represents the "thread" context
+    #[serde(rename = "thread")]
+    Thread,
+}

src/entities/mod.rs

@@ -6,6 +6,8 @@ pub mod attachment;
 pub mod card;
 /// Data structures for ser/de of contetx-related resources
 pub mod context;
+/// Data structures for ser/de of filter-related resources
+pub mod filter;
 /// Data structures for ser/de of instance-related resources
 pub mod instance;
 pub(crate) mod itemsiter;
@@ -39,6 +41,7 @@ pub mod prelude {
         attachment::{Attachment, MediaType},
         card::Card,
         context::Context,
+        filter::{Filter, FilterContext},
         instance::*,
         list::List,
         mention::Mention,