Support passing feature flags to cargo invocation

[encody]

Adds support for passing feature flags to cargo.

Motivation

Smart contract developers will sometimes use feature flags to enable/disable certain portions of code during compilation (e.g. enable certain debugging features for the testnet version which are not available in the production version).

Usage

cargo near build --features feat1,feat2

cargo near build --no-default-features

Implementation Notes

It does no validation of the input received to the flag, transparently forwarding it as an opaque string directly to cargo, after appending near-sdk/__abi-embed if necessary.

[frol]

@encody Nice! Looks good to me. @dj8yfo Do you have any objections for merging it (given the context of #134)

[frol]

@encody Hey, @dj8yfo flagged that this PR forgot to pass the feature flags to the ABI generation code:

cargo-near/cargo-near/src/commands/abi_command/abi.rs

Lines 87 to 89 in 358ce04

	let dylib_artifact = util::compile_project(
	&crate_metadata.manifest_path,
	&["--features", "near-sdk/__abi-generate"],

Could you contribute the fix, so this feature is complete?