Control which parts of files cannot be updated

[lukaszwawrzyk]

This issue is about fine grained control over updating files with versions. If for whatever reason it is undesired that bazel steward edits some version in some file (heuristics is wrong or anything), it should be possible to exclude some part of file from the detection/replacement mechanism.
We can use similar mechanism as linters/formatters do i.e. use comments like

block:

# bazel-steward off
MY_VERSION = "1.0.0"
MY_OTHER_VERSION = "1.0.2"
# bazel-steward on

inline

MY_VERSION = "1.0.3" # bazel-steward off

This is an extension of #61