Merge pull request #44 from staticfloat/sf/is_executable

Import `Base.Sys.isexecutable`

Author: staticfloat

Manifest.toml

@@ -14,6 +14,7 @@ export MagicMismatch
 # Import methods for extension
 import Base: seek, seekstart, skip, position, read, readuntil, eof,
              getindex
+import Base.Sys: isexecutable
 import StructIO: unpack
 
 """