Allow users to set single block params.

[cmrschwarz]

This just adds a simple method: set_block_param to the BlockContext.
It also makes the members of the (already public) PathAndJson struct public
so you can make effective use of it.

The existing set_block_params only allows replacing all block parameters.
Because there also is no get_block_params, this makes it impossible for a helper to
edit the block parameters effectively.

My usecase was to be able to write a helper that binds a variable to a name.
That helper is so useful that I added it to the examples aswell.

[coveralls]

coverage: 81.371% (-0.09%) from 81.461%
when pulling b176776 on cmrschwarz:set_block_parameter
into 3504412 on sunng87:master.

[coveralls]

coverage: 81.147% (-0.3%) from 81.461%
when pulling 43db374 on cmrschwarz:set_block_parameter
into 3504412 on sunng87:master.

[coveralls]

coverage: 81.191% (-0.3%) from 81.461%
when pulling 12b2f56 on cmrschwarz:set_block_parameter
into 3504412 on sunng87:master.