extend io::block::channel write capabilities

[niam]

Add such behaviour modifiers for write:

append
insert
replace
append truncate <- truncate source to pos and append, should be default for writeStream

[niam]

Insert will take too much resources for shifting data(especially for big files) so no much sense in implementation.

[niam]

If append modifier is false then absolute position(io::block::channel::pos) will be push the EOF. This operation is equal to append+truncate.