RFC/WIP for fread comment.char

[MichaelChirico]

Very simple version of how we could implement a comment.char argument for fread (#856). (just testing with a fixed comment.char='#' for now)

This addresses line-initial comments, but not line-trailing comments:

fread('a,b
#a comment
1,2
#another comment
3,4')
#       V1    V2
#    <int> <int>
# 1:     3     4

fread('a,b#line-trailing comment
1,2')
#        a b#line-trailing comment
#    <int>                   <int>
# 1:     1                       2

fread('a,b
1,2#trailing after numeric')
#        a                        b
#    <int>                   <char>
# 1:     1 2#trailing after numeric

Is this good enough to file as a solution for now? How common is the line-trailing use case? Any efficiency concern of this approach when comment.char is not used?

[MichaelChirico]

Superseded by #7375