remove undefined from options

[t-bltg]

The current solution: (builder.coffee l.98)

# Special case: no default options, no user options give [undefined]
if user_options.length==1 && user_options[0] == undefined
  user_options = []

for removing undefined has flaws: when directives.options is empty and user_options is not, then this lines yields

user_options = user_options.concat directives.option
# e.g. user_options = ['-shell-escape'] and directives.option = [undefined]
# results in user_options = ['--shell-escape', undefined] => undefined remains ...

as stated in #162

This is a fix based on filtering out any occurrence of undefined in the array user_options.

[r-stein]

Looks good to me