Add [once] table option to xshe.toml

[superatomic]

The [once] table should be a table that only runs if it hasn't setup any of its environment variables yet. It will do this by adding a environment variable __xshe_NAME, where NAME is the name of each environment variable.

For example, to set PATH without having it keep being modified every time xshe.toml gets sourced, this can be done:

[once]
PATH = ["$BIN_HOME", "$CARGO_HOME/bin", "$PATH"]

This will set these two environment variables if there is not a environment variable named $__xshe_PATH set to 1: $PATH to the expected value, and $__xshe_PATH to 1.

Additionally, specific tables [shell.SHELL.once] (where SHELL is any of the supported shells) should be created that work like shell.SHELL but only run if the given shell is being used.

[superatomic]

With the changes in #30, the necessity for shell-specific [once] tables is no longer relevant.

[superatomic]

v0.4.0 is shaping up to be a big release already, so I'm going to remove this issue from the v0.4.0 milestone. This issue will probably be implemented in v0.5.0.