Add yyextra to Lexer object

Add a yyextra field to allow passing a context object to the lexer.
This allows one to maintain state or to pass in additional information
required to interpret the lexed tokens.
Allow yyextra to be specified with a command-line flag --yyextra=whatever
Rename yyextraName -> yyExtraName and remove unecessary fmt.Sprintf
Merge pull request #2 from alexandercampbell-wf/yyextra_argv

Command-line parameter for yyextra
Merge pull request #1 from robertkluin-wf/yyextra

Add yyextra to Lexer object