fix: anchor the regex used for ictc (#17)

Rust's regex crate does not have a notion of `regex.match`; it instead
just provides `regex.find` and requires that you anchor your regex
yourself,
[discussion](rust-lang/regex#737).

This is more a correctness fix than a performance fix.

src/rules/if_change_then_change.rs

@@ -19,8 +19,8 @@ pub struct IctcBlock {
 }
 
 lazy_static::lazy_static! {
-    static ref RE_BEGIN: Regex = Regex::new(r"(?i) *(//|#) *ifchange").unwrap();
-    static ref RE_END: Regex = Regex::new(r"(?i) *(//|#) *thenchange (.*)").unwrap();
+    static ref RE_BEGIN: Regex = Regex::new(r"(?i)^\s*(//|#)\s*ifchange\s*$").unwrap();
+    static ref RE_END: Regex = Regex::new(r"(?i)^\s*(//|#)\s*thenchange\s*(.*)$").unwrap();
 
 }