Implement #7: Timestamp in log file name

[dvtomas]

Relevant configuration in file.toml:

# ...
timezone = "local" # utc or local

# {timestamp} will be replaced with timestamp in the appropriate time zone
# formatted according to the timestamp_template setting.
path = "file_{timestamp}.log"

# Format string for the timestamp in path.
# The string is formatted using [strftime](https://docs.rs/chrono/0.4.6/chrono/format/strftime/index.html#specifiers)
# Default: "%Y%m%d_%H%M", example: "20180918_1127"
timestamp_template = "%Y%m%d_%H%M"

[sile]

LGTM. Thanks!