Merge pull request bitfield#19 from udithprabhu/feature/tail

Added Last filter to mimic tail command of shell

Merge pull request bitfield#2 from SmilingNavern/add-list-files

Add ListFiles function

Fix typo

Merge pull request bitfield#11 from bitfield/column

Add Column() filter and Visitors example

Alphabetise methods

Have Concat skip errors

Add dummy tests to show code coverage

Add some example programs (cat, grep)

Make Exec() a no-op on erroneous pipe

Add Exec() filter